feat(design): UI design pass plan

docs/zeus-design.md

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+# UI design pass
+
+A user who opens Zeus for the first time should notice the chrome, not just the textbuffer. VS Code's UI is functional but visually anonymous — Zed's Rust + GPU + 120fps rendering has reset what "modern editor design" feels like.
+
+This is the design pass plan. Implementation is incremental; each section below becomes its own commit when work starts.
+
+## Three things to fix immediately
+
+1. **Title bar** — VS Code's default title bar is just a window chrome strip. Treat it as a real surface: project name on the left, current branch + sync state in the center, AI HUD on the right
+2. **Activity bar** — VS Code stacks icons vertically with no hierarchy. Group by function (files / search / git / agents / debug), separate visual treatment for the "AI agents" group
+3. **Command palette** — VS Code's palette is the most-used surface and the most boring. Larger, more typographic, with sectioned results and inline previews
+
+## Typography
+
+Monospace stack — OS-native first, JetBrains Mono (bundled with the app) as the cross-platform safety net:
+
+| OS | Primary | Fallback |
+|---|---|---|
+| macOS | SF Mono | JetBrains Mono (bundled), Menlo |
+| Windows | Cascadia Code | JetBrains Mono (bundled), Consolas |
+| Linux | Ubuntu Mono | JetBrains Mono (bundled), Hack, DejaVu Sans Mono |
+
+UI font (chrome, not the editor):
+
+- macOS: SF Pro Text → SF Pro Display for large headings
+- Windows: Segoe UI Variable
+- Linux: Inter (bundled) → system UI font
+
+JetBrains Mono (monospace) and Inter (UI) are bundled with the app so the experience stays consistent even when system fonts are unusual or non-Latin glyph coverage is missing. Bundled fonts sit one rung down from the OS-native primary so users who prefer their system font don't have it overridden silently — but on a fresh Linux desktop or a Windows machine without Cascadia, the bundled face still wins over a generic fallback.
+
+## Motion
+
+VS Code is mostly static. Zeus adds motion only where it earns its place:
+
+- **Focus change**: 120ms ease-out on the cursor blob position (caret carry effect)
+- **Command palette open**: 120ms fade-in with 4px translateY; never bounces (matches the focus-change duration above for visual consistency, and is long enough to read as a transition on 60Hz panels)
+- **Save**: 200ms checkmark pulse on the status bar item, no modal
+- **Agent completion**: brief glow on the parallel-agents status item, no sound
+- **No transitions on tab switch** — too high frequency, motion gets in the way
+
+All motion respects `prefers-reduced-motion`.
+
+## Themes
+
+Hand-tuned, shipped enabled:
+
+- **Zeus Light** — warm light, OLED-friendly contrast on black text
+- **Zeus Dark** — cool dark, accent gold (matches Z monogram in icon)
+- **Zeus High Contrast** — AAA contrast, distinct from VS Code's existing HC themes
+- **Zeus Dim** — between Dark and Light, for users who hate full dark
+
+Existing VS Code themes still install fine — these are additions, not replacements.
+
+## Iconography
+
+The Z monogram (lightning) becomes part of the visual language:
+
+- App icon (already present, `resources/zeus-icon.svg`)
+- Status bar "AI ready" indicator uses the same lightning glyph
+- Notification banner for agent completion has a small Z accent
+
+Avoid: putting the Z everywhere. Each appearance should mean something.
+
+## Status bar redesign
+
+Today's VS Code status bar is a tag soup. Zeus groups it:
+
+```text
+[ 🔀 main ↑2  ] [ ✦ Zeus  92% cache  $0.41 ] [ ⚠ 2 ]    [ Ln 12 · UTF-8 · Spaces:2 ]
+  ↑ left: git / project           ↑ middle: AI HUD     ↑ problems   ↑ right: editor state
+```
+
+Each cluster has its own visual treatment. Middle cluster is Zeus-specific. The `[ ⚠ N ]` chip is the existing vscode Problems indicator (errors + warnings count) pulled out of the right cluster so it sits next to the AI HUD — putting "the editor's diagnostics" and "the AI's diagnostics" side by side makes the relationship obvious.
+
+The cost segment (`$0.41`) is configurable via the `zeus.ai.hud.enabled` setting — users who don't want a live "ticker" can hide it without losing the rest of the HUD. The detailed HUD design (cache window, pricing source, persisted state) lands with `feat/prompt-cache-hud` (#28); once both PRs are merged this paragraph will link to `docs/zeus-prompt-cache-hud.md`.
+
+## Density modes
+
+Three density presets via `zeus.ui.density`, in descending order from tallest to most compact:
+
+- `comfortable` (default, tallest) — extra vertical breathing room vs VS Code
+- `cozy` (intermediate) — between `comfortable` and `compact`
+- `compact` — matches VS Code's default density
+
+## Open questions
+
+- Do we ship a custom monospaced font of our own (like Berkeley Mono vibe) or stick to per-OS native? Default: native.
+- Do we add a "showcase" page at first launch demonstrating the design? Conflicts with `feat/zero-config-defaults` ("no welcome page gates real work"). Probably no.
+- Animated app icon for "agents are running" — appealing but a fast way to look gimmicky. Hold.
+
+## Acceptance criteria
+
+- [ ] Title bar layout matches the spec in this doc
+- [ ] Themes ship enabled, not as marketplace prompts
+- [ ] All motion respects `prefers-reduced-motion`
+- [ ] Density toggle works without restart
+- [ ] Each section gets at least one before/after screenshot **when its implementation PR lands** (not in this design-only PR — nothing to screenshot yet)
+
+## Status
+
+Design only. Implementation is incremental; each section becomes its own commit + design review when work starts.