docs: add TSDoc documentation to User Settings module (#533)

[jadonamite]

Summary

• Adds comprehensive TSDoc/JSDoc documentation to the User Settings module (src/lib/settings/)
• Documents all AppSettings fields, store actions, and store state properties
• Adds a module-level usage guide with quick-start example, persistence notes, sync strategy, and export/import pointers

Files changed

src/lib/settings/types.ts

• Documents each AppSettings field: theme, language, notificationsEnabled, emailNotifications, prefetchingEnabled, reducedMotion
• Documents the export envelope schema and its migration intent
• Documents createDefaultSettings with rationale for each default value

src/lib/settings/store.ts

• Module-level JSDoc with a quick-start React snippet, persistence explanation (localStorage / SSR fallback), sync strategy using updatedAt as a vector clock, and export/import pointers
• Each store action documented: patchSettings (validation, trimming), replaceSettings (merge semantics), resetSettings (side-effects), setLastSyncedAt
• Store state fields documented: settings, updatedAt, lastSyncedAt

Test plan

• npm run type-check — no type errors
• npm run lint — no lint errors
• Existing export-import.test.ts still passes

Closes #533

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