Add desktop notifications and improve DM labeling

[wesbillman]

Summary

• add desktop notification preferences, Home badge state, and native desktop delivery for new mentions and needs-action items
• resolve generic DM labels to participant names and clear DM unread state from the latest loaded message
• add E2E coverage for notification settings, Home badge updates, and DM sidebar regressions

Testing

• source ./bin/activate-hermit && cd desktop && pnpm check
• source ./bin/activate-hermit && cd desktop && pnpm build
• source ./bin/activate-hermit && cd desktop/src-tauri && cargo check
• source ./bin/activate-hermit && cd desktop && pnpm exec playwright test --project=integration --grep "notification settings drive the Home badge and desktop alerts"
• source ./bin/activate-hermit && cd desktop && pnpm exec playwright test --project=smoke tests/e2e/channels.spec.ts

Addresses #100.