feat: auto-infer course language from user input

[cosarah]

Summary

• Replace the manual language selector (zh-CN / en-US toggle) with LLM-based automatic language inference
• A standalone /api/generate/language-directive endpoint infers a natural-language directive from requirement text, PDF content, user profile, and app locale
• The directive is consumed by agent profile generation, outline generation, and downstream content steps
• Add LLM-as-judge eval test suite (17 test cases, local-only)

Changes

• New: lib/generation/language-inference.ts — standalone inference function with dedicated prompt
• New: lib/types/language-directive.ts — type definition
• New: app/api/generate/language-directive/route.ts — API endpoint
• New: tests/generation/language-inference.eval.test.ts — eval suite with LLM-as-judge
• Modified: Remove manual language picker from generation toolbar
• Modified: Inject {{languageDirective}} into outline prompt (replaces {{language}})
• Modified: Propagate directive to agent profiles, interactive HTML, and Stage persistence
• Modified: Use app i18n locale instead of navigator.language for fallback signals
• Cleanup: Remove dead generationLanguage localStorage reads, orphaned i18n keys

Architecture

/api/generate/language-directive  (~1s, standalone LLM call)
  ↓ languageDirective (natural language string)
  ├→ /api/generate/agent-profiles  (language = directive)
  ├→ /api/generate/scene-outlines-stream  ({{languageDirective}} in prompt)
  └→ scene-content / scene-actions  (inherits via outline)

Test plan

• pnpm vitest run tests/generation/language-inference.eval.test.ts — 17/17 passing with LLM-as-judge
• Manual: generate course with Chinese requirement → verify Chinese output
• Manual: generate course with English requirement → verify English output
• Manual: generate course with Japanese requirement → verify Japanese output
• Manual: generate course with short ambiguous input + non-default app locale → verify locale fallback works

Closes #381

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