feat: add simplified cross-media visual themes

[xueyang-dev]

Summary

Adds the first version of simplified, cross-media visual themes. Teachers choose one visual direction once per project; the system then carries that decision into presentation, image, and video request metadata without exposing individual design parameters.

User experience

• Keeps the existing auto recommendation mode, with a deterministic reason shown to the teacher.
• Offers exactly five named themes: Classroom Clear, Active Learning, Warm Story, Eastern Elegance, and Future Exploration.
• Provides an accessible native dialog with explicit save/cancel behavior.
• Detects older media that does not match the selected theme and offers a guarded, backend-authorized way to unify it.
• Keeps unavailable video honest: the UI receives a styled request plan, never a fabricated video asset.

Architecture

• A versioned backend registry is the single source of truth for theme definitions, recommendation, availability, mismatch state, and available actions.
• Selection is persisted per project and supports legacy project data through safe normalization.
• Theme changes only invalidate downstream render/quality/delivery state when those artifacts exist.
• Media provenance distinguishes inherited historical assets from newly generated themed assets, so unknown old assets are not silently relabelled as current.
• Image-provider capabilities advertise visual-theme support; video remains an explicitly unsupported request contract in v1.

Scope cuts

• No theme editor, arbitrary palette/typography controls, template marketplace, or provider-specific visual configuration UI.
• No real video generation executor or claim that a provider can render video.
• No retroactive mutation of historical media; unification remains an explicit regeneration action.

Validation

• Focused backend theme tests: 37 passed (one existing FastAPI/Starlette deprecation warning).
• Full backend suite: 488 passed, 1 skipped, 7 warnings.
• Frontend state contracts, TypeScript check, and production build: passed.
• Playwright E2E: 8 passed.
• Remote GitHub Actions: both CI jobs passed.
• Real local-browser recheck used an isolated runtime at 1280×800 and 390×844: recommendation, manual selection, persistence after reload, mismatch/confirmation cancellation, focus restoration, and no horizontal overflow passed. Console warnings/errors were empty.
• git diff --check passed. The repository currently provides no lint or formatter script.

Known limitation

The current browser-host injection channel remains unavailable because it attempts to redefine process; the browser acceptance was completed with the repository’s local Playwright/Chrome stack instead. This PR does not add a real video executor.

Stacked PR

This is a focused stacked PR with base codex/zero-beginner-courseware-quality, rather than main, so the six prior zero-beginner courseware commits are not mixed into the visual-theme review. Its parent work ultimately depends on Draft PR #22.

Review focus

• Registry/API authority, persistence, and legacy normalization.
• Downstream invalidation and mismatch semantics.
• No fake availability or video output.
• Dialog accessibility and responsive behavior.
• Tests around historical-media provenance and selection persistence.