Customizable app theme (built-in palettes + CLI custom stylesheet)

[SawyerHood]

Summary

Makes the app's color palette customizable — server-stored and applied live to every open window, orthogonal to the per-client light/dark mode.

• Backend: single-row app_theme table + appThemeSchema; surfaced on GET /system/config and a new PUT /settings/appearance; broadcast via config-changed (migration 0042).
• CLI: bb theme list | set <id> | set-custom --file <css> | show [--css] | reset (new @bb/sdk theme area).
• Frontend: built-in palettes (nord, dracula, solarized, gruvbox) as a CSS-string registry; useAppTheme injects a trailing <style> (wins the cascade); pre-paint cache avoids a flash; Settings → Appearance palette picker.

Make existing chrome follow the tokens

• --surface-scrim derives from --canvas, so the frosted header bar tracks the palette (was hardcoded white).
• PullRequestStatusPill: GitHub status hexes → text-success/-destructive/-pr-merged/-muted-foreground.
• Mermaid diagrams resolve their palette from live CSS vars (probe + canvas rasterize to sRGB) and re-render on palette change via a new useAppThemeEpoch; the terminal re-themes on the same signal.
• Built-in themes set --ansi-bg-fg-* contrast colors.

Docs

• bb-cli skill gains references/theming.md (token model, what each variable drives, colors, fonts), pointed to from SKILL.md.

Tests

• New apps/server/test/system/appearance.test.ts (default, set built-in, set custom, reject unknown id, reject custom-without-css).
• migrate.test.ts rewind helper updated to drop app_theme.
• Local: typecheck 7/7 touched packages; server (17), db migrate (22), app theme (16) tests pass.
• Verified live in the dev app: all 5 built-ins in light + dark (headers coordinate, text contrast AA+); Mermaid re-renders live on theme switch.

Risks / notes

• Custom CSS is injected raw (capped 256 KB), no sanitizer — acceptable for a local single-user tool.
• Solarized dark is intentionally low-contrast (still AA).

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