fix: validate all provider selections after server sync

[wyuc]

Summary

• Extract isProviderUsable, validateProvider, validateModel into lib/store/settings-validation.ts
• Replace PR fix: clear stale modelId/providerId after server provider sync #232's LLM-only inline validation with unified validation for all 6 provider types (LLM, TTS, ASR, PDF, Image, Video)
• Usability checks isServerConfigured || apiKey — does NOT use requiresApiKey (e.g. mineru needs server baseUrl, not API key)
• Auto-select fallback instead of clearing to empty: TTS→browser-native-tts, ASR→browser-native, PDF→unpdf
• Image/Video: auto-recover provider+model from empty state when server adds providers; auto-disable generation when all providers removed; prevent re-enabling until a provider becomes available
• No auto-enable on recovery — user controls when to turn on generation
• PDF providers now recognized with baseUrl-only config (requiresBaseUrl option in loadEnvSection)
• Remove "setup needed" prompt from header and home page (auto-select makes it unnecessary)
• Clean up dead setup-indicator CSS animations

Closes #263

Test plan

• 16 unit tests for validation functions (isProviderUsable, validateProvider, validateModel)
• 34 integration tests for stale selection across all provider types
• 23 server provider-config tests including baseUrl-only PDF scenarios
• 74 total tests pass, typecheck clean, lint clean
• Manual verification: provider add/remove/restore cycles for LLM, Image, TTS

🤖 Generated with Claude Code

[cosarah]

Issues

Important

settings.ts:925-939 — Fallback arrays for TTS/ASR/PDF/Image/Video only include server-configured providers, while LLM's fallback also includes providers with client-side API keys. This means if a user manually sets a client API key for a non-LLM provider (e.g. OpenAI TTS) and the server config for that provider is later removed, the fallback will skip it and force-switch to browser-native — even though the client key is still valid. Consider unifying the fallback strategy across all provider types.

Minor

• settings.ts:1150-1171 — Auto-select spreads after validation on first load, silently overriding validated results. A comment documenting the intended precedence would help maintainability.
• settings-validation.ts:30 / settings.ts:977 — validateProvider('') short-circuits to '', forcing auto-recover to duplicate the "pick first fallback" logic.
• settings.ts:1000-1003 — || imageModels[0]?.id is redundant with validateModel's own fallback.

Summary

Solid PR — clean extraction of validation helpers, unified approach for all 6 provider types, thorough test coverage. Main concern is the inconsistent fallback strategy for non-LLM providers that could drop valid client-key configurations.

[cosarah]

All previous review items addressed:

• Fallback strategy unified via buildFallback<T>() — all 6 provider types now include both server-configured and client-key providers. Core issue resolved.
• Auto-select override precedence documented with clear comments.
• Redundant imageModels[0]?.id fallback commented.

Clean fix. LGTM.