feat(mobile): add Smart Actions — API client, prompt store, note detail integration
Phase 4 of Smart Actions Roadmap:
- Create mobile/src/api/note-prompts.ts: typed API client
- listPromptTemplates, runPrompt, suggestTags, getReadingTime
- Create mobile/src/store/prompt-store.ts: Zustand store
- fetchTemplates, runPrompt, clearResult, clearError
- Enhance note detail screen (mobile/src/app/note/[id].tsx):
- Collapsible Smart Actions section with toggle
- Reading time display on expand
- Suggest tags button with inline tag display
- Prompt template list (top 6) with one-tap execution
- ActivityIndicator during prompt execution
- Inline result card with model/token info and dismiss
- Mobile typecheck passes
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