Store custom app themes under .bb/theme/<name>/theme.css

[SawyerHood]

Summary

Moves custom app-theme storage out of the database and onto disk, following the BB .bb convention: custom themes now live under <data-dir>/theme/<name>/theme.css (the packaged app uses ~/.bb/theme/…), mirroring how user skills live under <data-dir>/skills/. The folder name is the palette id.

This is a breaking change (as requested): the old bb theme set-custom --file path and the DB-stored custom CSS blob are removed. Custom themes are now created/edited by writing the file and selected by name.

Behavior

• Discovery/loading: the server scans <data-dir>/theme/*/theme.css, resolves the active theme's CSS from disk, and broadcasts it live. A selected theme whose folder is deleted falls back to the default palette.
• Server: appearance in /system/config is resolved server-side; added customThemes to the config and a new GET /settings/themes catalog ({dir, custom, active}). PUT /settings/appearance now takes a theme-id selection and validates it (400 for an unsafe id, 404 for a missing custom theme, with the expected path in the message).
• CLI: theme list enumerates built-ins + discovered custom themes + the dir; theme set <id> accepts a custom theme name; new theme dir; removed theme set-custom.
• Settings: the Palette picker lists discovered custom themes alongside built-ins.
• DB: dropped the now-unused app_theme.custom_css column (migration 0043); the table stores only the active selection.
• Docs/skills: updated the bb-cli skill + theming reference to establish the .bb/theme/<name>/theme.css habit and the create/edit/select workflow.

Tests

• @bb/db — 307 passed (incl. migration journal/folder checks validating 0043).
• @bb/server theme tests — 13 passed (appearance + new custom-themes service: discovery, disk resolve, delete-fallback, size cap, 404 missing, 400 unsafe name).
• Typecheck (Turbo): @bb/domain, @bb/db, @bb/server-contract, @bb/sdk, @bb/server, @bb/app, @bb/desktop — all clean.
• Manual QA: ran the dev server, verified the catalog endpoint, customThemes in config, the picker lists a sample sunset theme, and selection applies live.

Notes / risks

• Custom theme names are reserved against built-in ids and constrained to a safe single path segment (no ../separators); CSS is size-capped to keep the broadcast payload bounded.
• Pre-existing unrelated typecheck failure in apps/cli/.../thread-open.test.ts (missing activity field from Change thread open to resolve workspace paths #276) — confirmed present on a clean tree, untouched here.

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