feat(companion): backup & restore + NIP-46 remote signer — 0.5.28 (#128, #139)

Companion 0.5.28 (versionCode 48), the companion-agent queue items:

#128 Backup & Restore — the phone side of losing your phone or wiping it
to cross a border. Hub card → SAF export/import of an encrypted .json:
everything the app holds (servers+passwords, FIPS identity/peers, signer
key) sealed in the node's ADR-005 envelope (Argon2id + ChaCha20-Poly1305,
native backup.rs — same blob layout as the node's, node-shaped envelopes
decrypt too). Restore is merge-only: servers upsert npub-first, identity
and signer key adopt only when absent, peers union by npub. No cloud, no
telemetry — the file goes wherever the user saves it.

#139 Remote Signer — the phone IS the NIP-46 bunker. Generate/import a
nostr key, scan a nostrconnect:// QR (in-app scanner or deep link), and
approve/deny each sign_event request from a legible card (kind label,
content, tags, time) — nothing signs without a human. Wire-faithful to
rust-nostr's reference bunker (connect-carrying-secret handshake, NIP-44
v2 transport with NIP-04 receive fallback, kind-24133 responses);
get_public_key/describe/ping handled, everything else 'not authorized'.
Session state in BunkerManager, UI in SignerScreen, hub card wired.

Plus NativeCore (JNI object for the new native surface), FipsPreferences
peers-merge for restore, nostrconnect:// intent filter, and the release
docs (companion-backup-restore.md, companion-nip46-remote-signer.md).

Also Android/tools/nip46-test-client.py: a pure-Python NIP-46 client that
plays the node's login role (QR, handshake, get_public_key, sign_event)
and verifies the phone's signature with an independent BIP-340 — the
end-to-end test for the feature until node-side lands. Its crypto matches
the official NIP-44 + BIP-340 vectors byte-for-byte, the same vectors the
Rust core passes, so the two interop by construction.

Built + smoke: assembleDebug v0.5.28/vc48, same signing cert as the
served 0.5.27 (d622e07e…644d) so it updates in place.

Android/app/src/main/AndroidManifest.xml

@@ -54,6 +54,15 @@
                 <category android:name="android.intent.category.BROWSABLE" />
                 <data android:scheme="archipelago" android:host="pair" />
             </intent-filter>
+            <!-- Remote-signer pairing deep link (NIP-46, companion 0.5.28):
+                 nostrconnect://<client-pubkey>?relay=...&secret=... — the
+                 node's login QR, hand-off from any QR scanner app. -->
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data android:scheme="nostrconnect" />
+            </intent-filter>
         </activity>
 
         <!-- Embedded FIPS mesh node: split-tunnel VpnService (fd00::/8 only),