Add Tailwind CSS setup, theme system, and UI components / flow

frontend/fixme.md

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+# Fix List (CodeRabbit Review)
+
+## Bugs — fix immediately
+
+- [ ] `League.tsx` — add null check before `Object.hasOwn(leagueJson, 'user_id')`, throws if fetch returns null
+- [ ] `CreationModal.tsx` — race condition on `errors` state in `handleSubmit`, use local tracking variable instead of reading stale state
+- [ ] `CreationModal.tsx` — `validateUnique` is passed `value` instead of `fieldName` as second argument
+- [ ] `useResource.ts` — guard `response.items ?? []`, undefined will break rendering
+- [ ] `Leagues.tsx` — rename reloads even if API call failed, check response before calling `setReloadKey`
+- [ ] `DisplayResourceGeneric.tsx` — add button never shows when `totalPages === 0`, condition should be `totalPages === 0 || pageNum === totalPages`
+
+## Defensive — good practice
+
+- [ ] `fetchLeaguesIndex.ts` — add `response.ok` check, 4xx/5xx silently proceed to `.json()`
+- [ ] `fetchLeaguesSpecific.ts` — same missing `response.ok` check
+- [ ] `renameLeague.ts` — same missing `response.ok` check
+- [ ] `errorSlice.ts` — replace `??` with `||` to catch empty string payloads
+- [ ] `ProtectedRoutes.tsx` — add cleanup to `useEffect` so loader doesn't get stuck on unmount
+
+## Polish
+
+- [ ] `ExtendableContextMenu.tsx` — hardcoded `white`/`black` colours, swap for `var(--color-bg-card)` and `var(--color-border)`
+- [ ] `ExtendableContextMenu.tsx` — tautological ternary `'Submit' : 'Submit'`, change to `'Validating...' : 'Submit'`
+- [ ] `CreationModal.tsx` — tautological ternary `'Create' : 'Create'`, change to `'Validating...' : 'Create'`
+- [ ] `ThemeTest.tsx` — initial dark state hardcoded to `true`, should read from DOM: `!document.documentElement.classList.contains('light')`
+
+## Skipped
+
+- `App.tsx` route comments — intentional, ignore
+- `ErrorBanner.tsx` aria label — defer to accessibility pass