feat(frontend): add owner-scoped reminder store

[yimi528]

Summary

• Add FrontendReminderStore as the persistence boundary for frontend-owned reminders.
• Keep reminder records scoped by owner and validate text, timestamps, IANA time zones, kinds, recurrence, and lifecycle states.
• Persist versioned JSON atomically with private permissions and a shared file transaction lock.
• Recover an interrupted firing reminder as active so a later runtime can retry it.
• Add due inspection, atomic due claiming, cancellation, completion, recurring rescheduling, and failure transitions.

This is the storage-only T1 slice from the frontend Agent capability roadmap. It intentionally does not wire new tools, the existing schedule_reminder path, ReminderScheduler, AnnouncementManager, or BackendPort.

Data safety

• Reads and writes are owner-scoped; an owner cannot inspect or cancel another owner's reminder.
• Invalid or corrupt records are skipped or quarantined without taking down the Gateway.
• Failed persistence rolls back the in-memory mutation.
• No credentials, audio, logs, or backend session data are stored.

Validation

• node --test server/test/frontend-reminders.test.mjs (11 tests passed)
• eslint server/src/conversation/frontend-reminders.mjs server/test/frontend-reminders.test.mjs
• git diff --check

Follow-up PRs can connect this store to reminder tools and scheduling after the data contract is reviewed.