Add BACKLOG.md with prioritized improvement tasks for future agents

BACKLOG.md

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+# BACKLOG.md — Improvement Tasks for Future Agents
+
+This document captures prioritized, actionable improvement tasks identified
+through a comprehensive code analysis. Each task includes the problem, affected
+files/lines, and a concrete implementation plan.
+
+**Ground rules for agents working on these tasks:**
+- Run `node .claude/hooks/run-tests.js` before and after every change
+- Add tests in `tests.html` for any new logic
+- Keep the zero-dependency, no-build-step constraint
+- Do not add npm packages or bundlers
+- All JS must work without transpilation (no JSX, no ES modules)
+
+---
+
+## Task 1 — Add React Error Boundary and Crash Recovery
+
+**Priority:** HIGH | **Effort:** Low | **Category:** Reliability
+
+**Problem:** No error boundary exists. If any React component throws, the entire
+app goes blank with no recovery path. Users must hard-refresh and may not
+understand what happened.
+
+**Files:**
+- `index.html` — wrap App component (around line 1295)
+
+**Implementation:**
+1. Add an `ErrorBoundary` class component before the App definition:
+   ```js
+   class ErrorBoundary extends React.Component {
+     constructor(props) { super(props); this.state = { hasError: false, error: null }; }
+     static getDerivedStateFromError(error) { return { hasError: true, error: error }; }
+     componentDidCatch(error, info) { console.error('App crash:', error, info); }
+     render() {
+       if (this.state.hasError) {
+         return React.createElement('div', { style: { padding: 40, textAlign: 'center' } },
+           React.createElement('h2', null, 'Something went wrong'),
+           React.createElement('p', null, this.state.error && this.state.error.message),
+           React.createElement('button', { onClick: function() { location.reload(); } }, 'Reload'),
+           React.createElement('button', { onClick: function() { localStorage.clear(); location.reload(); } }, 'Reset all data')
+         );
+       }
+       return this.props.children;
+     }
+   }
+   ```
+2. Wrap `<App />` in `<ErrorBoundary>` in the `ReactDOM.createRoot` call
+3. Add a test: render ErrorBoundary with a child that throws, assert fallback UI appears
+
+---
+
+## Task 2 — Handle localStorage Quota/Disabled Errors
+
+**Priority:** HIGH | **Effort:** Low | **Category:** Data Safety
+
+**Problem:** `lsSave` (line ~309-312) and `srsSave` (line ~472-476) silently
+catch exceptions. If localStorage is full or disabled, users lose progress with
+no warning.
+
+**Files:**
+- `index.html` — `lsSave`, `srsSave`, and all `localStorage.setItem` calls
+
+**Implementation:**
+1. Create a `safeSave(key, value)` helper that wraps `localStorage.setItem` in
+   try/catch and returns `{ ok: boolean, error: string|null }`
+2. On failure, show a non-blocking notification bar at the top of the page:
+   "Unable to save progress — storage may be full or disabled"
+3. Add a `storageAvailable()` check at app startup; show a persistent warning
+   if localStorage is not available
+4. Replace all direct `localStorage.setItem` calls with `safeSave`
+5. Add test: mock `localStorage.setItem` to throw, verify `safeSave` returns error
+
+---
+
+## Task 3 — Add Accessibility Attributes
+
+**Priority:** HIGH | **Effort:** Medium | **Category:** Accessibility
+
+**Problem:** No `aria-label`, `role`, or `tabindex` attributes on interactive
+elements. Screen reader users cannot navigate the app.
+
+**Files:**
+- `index.html` — all React component `createElement` calls
+
+**Implementation (incremental — can be split across multiple PRs):**
+1. **Speak buttons:** Add `aria-label="Listen to pronunciation"` to all TTS buttons
+2. **Quiz buttons:** Add `role="button"` and `aria-label` with the answer text
+3. **Feedback messages:** Add `aria-live="polite"` to exercise result text
+4. **Navigation:** Add `role="navigation"` to header, `role="main"` to content area
+5. **Review cards:** Make card flippable via Enter/Space key (add `tabIndex={0}`
+   and `onKeyDown` handler)
+6. **Overview calendar:** Add `aria-label` to day cells with day number and status
+7. Verify with a screen reader or axe-core
+
+---
+
+## Task 4 — Sanitize SVG in dangerouslySetInnerHTML
+
+**Priority:** MEDIUM | **Effort:** Low | **Category:** Security
+
+**Problem:** Line ~579 uses `dangerouslySetInnerHTML` for stroke order SVGs
+fetched via `fetch()`. While currently fetched from a known source, this is a
+latent XSS vector.
+
+**Files:**
+- `index.html` — `CharCard` component (line ~579), `loadStrokeOrderSvg` (line ~343)
+
+**Implementation:**
+1. Add a `sanitizeSvg(raw)` function that:
+   - Parses with DOMParser
+   - Removes `<script>`, `on*` attributes, `<foreignObject>`, `<use>` with external hrefs
+   - Returns the cleaned SVG string
+2. Call `sanitizeSvg` on the fetch result before passing to `dangerouslySetInnerHTML`
+3. Add a Content-Security-Policy `<meta>` tag to `index.html`:
+   ```html
+   <meta http-equiv="Content-Security-Policy" content="default-src 'self' https://cdnjs.cloudflare.com; script-src 'self' 'unsafe-inline' https://cdnjs.cloudflare.com; style-src 'self' 'unsafe-inline';">
+   ```
+4. Add tests: pass SVG with `<script>` tag, verify it is stripped
+
+---
+
+## Task 5 — Graceful SVG Network Failure Handling
+
+**Priority:** MEDIUM | **Effort:** Low | **Category:** UX
+
+**Problem:** `loadStrokeOrderSvg` (line ~343-350) has a `.catch()` that sets
+error state but provides no retry, no fallback visual, and no caching.
+
+**Files:**
+- `index.html` — `loadStrokeOrderSvg` function, `CharCard` component
+
+**Implementation:**
+1. Add retry logic: 2 retries with 1s/2s delays
+2. Cache successful SVGs in `sessionStorage` (key: character code point)
+3. On final failure, show the character in large font with text "Stroke order
+   unavailable" instead of a blank area
+4. Add a "Retry" button in the error state
+
+---
+
+## Task 6 — Runtime Curriculum Validation
+
+**Priority:** MEDIUM | **Effort:** Low | **Category:** Reliability
+
+**Problem:** Tests validate curriculum structure, but no runtime check exists.
+If `curriculum.js` is corrupted or partially loaded, the app silently breaks.
+
+**Files:**
+- `index.html` — App component init
+- `lib.js` — add `validateCurriculum()` function
+
+**Implementation:**
+1. Add `validateCurriculum()` in `lib.js`:
+   - Check `curriculum.length === 1720`
+   - Spot-check first/last/boundary days have required fields
+   - Return `{ valid: boolean, error: string }`
+2. Call at App mount; if invalid, render an error message instead of the app
+3. Add test for `validateCurriculum` with valid and corrupted data
+
+---
+
+## Task 7 — Consolidate Duplicate Review Button Styles
+
+**Priority:** LOW | **Effort:** Low | **Category:** Code Quality
+
+**Problem:** Review button styles are defined in two places (lines ~157-160 and
+~181-185) with slightly different values. The SRS grade buttons also have
+hardcoded inline styles.
+
+**Files:**
+- `index.html` — `<style>` block
+
+**Implementation:**
+1. Audit all `.review-btn` and grade button styles
+2. Consolidate into a single CSS class hierarchy
+3. Remove duplicate declarations
+4. Extract hardcoded level colors (line ~1253: N5=#e91e8c, etc.) into CSS custom
+   properties that reference the `PHASE_COLORS` constants
+
+---
+
+## Task 8 — Add TTS Rate and Voice Controls
+
+**Priority:** LOW | **Effort:** Low | **Category:** UX
+
+**Problem:** TTS rate is hardcoded to 0.85 (line ~322). Learners at different
+levels need different speeds. No voice selection is available.
+
+**Files:**
+- `index.html` — `speak()` function (line ~316-333), App header
+
+**Implementation:**
+1. Add a `speechRate` state (default 0.85) persisted to localStorage
+2. Add a small dropdown or slider in the header: 0.5x / 0.75x / 1.0x / 1.25x
+3. Pass `speechRate` to `utterance.rate` in the `speak()` function
+4. Optionally enumerate available Japanese voices and let user pick
+
+---
+
+## Task 9 — Add Export/Import Progress
+
+**Priority:** LOW | **Effort:** Medium | **Category:** UX
+
+**Problem:** All progress is in localStorage with no backup mechanism. Users
+lose everything if they clear browser data.
+
+**Files:**
+- `index.html` — add Export/Import buttons to Overview or header
+
+**Implementation:**
+1. Add "Export Progress" button that:
+   - Collects all `jlpt_*` keys from localStorage
+   - Creates a JSON blob with version field
+   - Triggers a file download (`jlpt-progress-YYYY-MM-DD.json`)
+2. Add "Import Progress" button that:
+   - Opens a file picker for `.json`
+   - Validates the structure
+   - Confirms overwrite with user
+   - Writes to localStorage and reloads
+3. Add tests for the export/import data format validation
+
+---
+
+## Task 10 — Input Validation Hardening in checkTyping
+
+**Priority:** LOW | **Effort:** Low | **Category:** Quality
+
+**Problem:** `checkTyping()` (lib.js line ~449-461) doesn't handle edge cases:
+excessively long input, Unicode normalization differences, or mixed-script input.
+
+**Files:**
+- `lib.js` — `normAns`, `checkTyping`
+
+**Implementation:**
+1. Add max-length guard (200 chars) — reject as incorrect
+2. Apply Unicode NFC normalization in `normAns`
+3. Add tests for: very long input, combining characters, fullwidth/halfwidth
+   romaji equivalents
+
+---
+
+## Completed Tasks
+
+_Move tasks here as they are finished, with the date and a one-line summary._