Fix correctness, security, and concurrency issues from code review

Addresses findings from a full-repo review across the mail, sync, persistence,
auth, and UI layers.

Send / outbox:
- Stop the Graph->SMTP fallback from duplicating a message when a Graph send may
  already have been accepted; leave indeterminate sends queued for the user.
- Parse RFC822 display-name recipients on the Graph path.
- Preserve attachment order (staged in indexed subdirectories).

Data safety (schema v7):
- Disable cloud/device backup of the Keystore-encrypted credential DB.
- Add the missing v1->v2 migration and drop the destructive migration fallback.
- Normalize the messages.isHtml default and add an attachments->messages
  ON DELETE CASCADE foreign key (no more orphaned attachment rows).

Concurrency:
- Serialize syncing and per-account OAuth token refresh; cache tokens by expiry.
- Synchronize Android Keystore key creation.
- Make a sync's persist+notify non-cancellable so an IDLE renewal can't drop it.

Notifications / UI:
- Per-message notifications under a group + summary instead of one overwriting id.
- Wire the "load remote images" and "allow STARTTLS" settings.
- Harden the reader WebView (scheme allowlist + user gesture; no reload on
  recomposition); refresh headers without reverting optimistic read/star flags.
- Persist draft attachments; keep server-search hits out of the inbox; encode
  the reader navigation argument.

Build / test:
- Export Room schemas; support a real release keystore; add unit/db tests.
- Remove dead Gmail account-setup code left after the sign-in removal.

Verified: debug + release (R8) + androidTest compile; unit tests pass;
MIGRATION_6_7 matches the generated v7 schema.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -11,7 +11,8 @@
 
     <application
         android:name=".LibreMailApplication"
-        android:allowBackup="true"
+        android:allowBackup="false"
+        android:dataExtractionRules="@xml/data_extraction_rules"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"