feat: per-conversation notification settings

[coda-rho-bot]

Summary

• Per-conversation notification settings: All messages (every run completion, any client), Mobile only (runs started from this app), Off — selected from a sheet behind a composer chip (🔔), persisted in AsyncStorage, resolved against the app-wide default.
• ChatSession.onRunCompleted callback: fires once per turn — local result frames and external-run loop_status idle transitions — so the UI can post a system notification gated by mode + app visibility.
• Notification permission requested once per install (no nagging after denial).
• Groundwork for background run-completion notifications (an always-on native poller builds on this and arrives separately).

Test plan

• Chip shows the resolved mode; sheet selection persists across app restarts
• Off mode posts nothing; Mobile only posts for runs started on-device
• Notifications fire only while the app is backgrounded
• TypeScript-clean on the branch; Android CI validates the full build

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• I am the author of these changes, with the assistance of AI tools
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• AI tools used: Letta Code agent (GLM-5.3 and GLM-5.3 Flash via z.ai, per provider usage records)
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