Add a per-account settings area (reached by tapping an account in
Settings), starting with signatures and notifications.
- Storage: new Room `account_settings` table (schema v9, MIGRATION_8_9)
with a cascading foreign key to `accounts`; AccountSettings model and
AccountSettingsRepository (a missing row resolves to defaults).
- Signatures: plain-text per-account signature (RFC 3676 "-- "
delimiter) auto-inserted below new messages and reply/forward drafts,
and swapped when the From-account changes.
- Notifications: one notification channel + channel group per account so
Android manages sound/vibration/importance (deep-linked from the app)
and the shade bundles per account, plus an in-app per-account on/off
gate. minSdk 29 >= API 26, so channels are always available (no
pre-channel fallback needed).
- UI: per-account settings screen (signature field/toggle, notification
toggle, system deep-link, remove account); shared settings components.
Verified: JVM unit tests, lintVitalRelease (NewApi), and the full
instrumented suite (30/30) on the API 37 emulator.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
