android: the device token is stored sealed under a Keystore key

Family idea #5105, practice 12, as the operator chose on 2026-10-08: the token
is encrypted, and Android backup stays on.

The core:
- Adds a TokenSeal trait in sync/state.rs, with set_sealed_link and
  open_token.
- A sealed token is stored as "sealed:<value>".
- A plain token, stored before this change or while sealing failed, is sealed
  in place on its next read.
- A sealed token that won't open is dropped, and the server address and cursor
  are kept, so the app reads as unlinked and asks to sign in again. That is
  what happens after Android restores the app onto another phone.
- The desktop passes no seal and keeps storing the token as before.

The FFI:
- Exports TokenSeal as a uniffi foreign trait (seal_token / open_token, null
  rather than an exception).
- Requires it in Inkwell's constructor, so there is no moment a token could be
  stored unsealed.
- Routes credentials(), unlink() and store_link() through it.

Kotlin:
- KeystoreTokenSeal is AES-GCM under an Android Keystore key, using the
  SealedBox framing from Minstrel's KeystoreSessionVault (Scribe snippet #5025),
  with no new dependency.
- SealedBoxTest checks the framing on the JVM.

allowBackup stays true, and the manifest says why. An unlinked phone's notes
exist only on the phone, and the backup is their one other copy. The backup
carries a token nothing can open.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/src/main/AndroidManifest.xml

@@ -90,6 +90,14 @@
     <uses-permission android:name="android.permission.SCHEDULE_EXACT_ALARM" />
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
 
+    <!--
+      allowBackup stays true, which family idea #5105 (practice 12) says to turn
+      off. Inkwell is local-first, so an unlinked phone's notes exist only on the
+      phone, and Android's backup is their one other copy. The device token is what
+      the practice protects, and it is stored sealed under a Keystore key that never
+      leaves the phone (KeystoreTokenSeal.kt). A backup therefore carries the notes
+      and a token nothing can open, and a restored app asks to sign in again.
+    -->
     <application
         android:name=".InkwellApplication"
         android:allowBackup="true"