feat(web): add canonical theme preference

[testikun]

Problem

OpenPI Web had no package-owned theme preference. A browser-local choice would create a second source of truth and drift from the canonical /openpi-setup contract. This implements the canonical theme slice of #350 without overlapping PR #359's language/localStorage work.

Value

Users can choose system, light, or dark once through OpenPI setup, see that choice in no-argument setup status, and have every Web client render the same authoritative preference. The default follows the browser/OS color scheme.

Approach

• Add ui.webTheme to MyPiSetupConfig, defaulting and failing closed to system.
• Extend the episode-scoped typed setup writer with ui_web_theme, update status/prompt/docs/defaults, and register the field in the config drift guard.
• Project the canonical preference in WebSnapshot; the browser resolves system with prefers-color-scheme and never persists a competing theme value.
• Refactor Web colors into semantic light/dark tokens so interactive, code, menu, dialog, and composer states remain coherent.

Validation

• bun run check — passed, including the 15-field configuration contract.
• bun run test — passed: Node 1241 passed / 1 platform skip; Vitest 30 passed.
• Focused setup/config/Web suites — 95 passed.
• Runtime provenance: isolated pi list reported the single OpenPI source /tmp/openpi-issue-350, whose checkout was based on 72fbba52832841cfc78c2f7e7947eeb89efc73bc plus this PR commit.
• Real local Workbench test: canonical ui.webTheme=dark produced preferences.theme=dark from /api/snapshot and rendered the final dark UI in an actual browser. No mock image was used. The 1280×720 screenshot is attached to the first feat(web): add persistent theme/language preferences and accessibility/mobile hardening #350 claim comment.

Impact

• User-visible behavior: OpenPI Web gains canonical system/light/dark themes; default remains system-following.
• Model-visible context/tools: the existing setup tool gains one bounded enum field only during setup episodes; no ordinary-turn tool is added.
• Runtime/lifecycle: snapshots synchronously read the small canonical setup document; system mode reacts to browser color-scheme changes for the page lifetime.
• Persisted config/data: adds optional canonical ui.webTheme; old or invalid documents read safely as system and are normalized on the next setup write.
• Compatibility/risk: additive protocol field with protocol version unchanged; older clients ignore it, and the new client safely falls back to system if absent.