fix(android): stop backing up credentials no key can ever open

The app's data dir was eligible for Google cloud backup: the manifest set
neither allowBackup nor any extraction rules, so the SQLite catalogue
(library metadata, watch history) and the jellytau_secure_prefs
credential blob were shipped to the user's Google account. Restoring that
is worse than not having it — SecureStorage encrypts under an Android
Keystore key, and Keystore keys are never backed up, so a restored
install gets ciphertext with nothing to open it and fails auth silently
while looking signed in.

Backup and device-to-device transfer are both turned off. allowBackup
="false" covers API 24-30 outright and kills cloud backup on 31+; it does
NOT stop D2D there, so @xml/data_extraction_rules excludes every domain
from both channels. Nothing is lost: the catalogue is a rebuildable
mirror of the Jellyfin server, and watch state lives on the server.

The credential-load path degrades instead of erroring, because a device
can still arrive at undecryptable ciphertext (an older install's backup,
a Keystore key invalidated by a lockscreen change). Both backends now
distinguish "nothing stored" from "stored but unreadable" and answer the
second as the first: CredentialStore::load_credentials_file logs and
returns an empty map rather than CredentialError::Encryption — which
storage_get_access_token was turning into a hard Err and
storage_get_active_session into a warning — and SecureStorage.getCredential
discards the dead blob so it cannot fail every subsequent read. The
result is a login screen rather than a broken session, and the next
successful sign-in rewrites the store.

Also removes the half-declared Android TV support: the manifest offered
LEANBACK_LAUNCHER and the leanback uses-feature with no D-pad focus
model, no TV layouts, and neither of the two declarations Play's TV
validation also requires (touchscreen required="false", android:banner).
That fails review while advertising the app to TV launchers. All four go
back together when a focus pass is actually done.

And raises jvmTarget from 1.8 to 17 under compileSdk 36, with matching
compileOptions — AGP 8.11 already requires a JDK 17 toolchain, so 1.8 was
only capping emitted bytecode. Nothing else in the build assumed 1.8.

TRACES: UR-012 | IR-014

src-tauri/android/src/main/res/xml/data_extraction_rules.xml

@@ -0,0 +1,45 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  Backup / transfer policy for JellyTau (API 31+; see android:allowBackup in
+  AndroidManifest.xml for API 24-30).
+
+  Nothing is eligible for extraction, from either channel:
+
+  * cloud-backup   - already off via android:allowBackup="false".
+  * device-transfer - NOT covered by allowBackup on Android 12+, which is why
+    this file exists. A D2D transfer would otherwise copy the same data the
+    cloud backup used to.
+
+  Why nothing is extractable:
+
+  * Credentials are unrecoverable off-device. jellytau_secure_prefs holds
+    AES-GCM ciphertext encrypted under an Android Keystore key, and Keystore
+    keys are never backed up or transferred. Restoring the prefs without the
+    key produces ciphertext nothing can read - a silent auth failure that looks
+    like a broken app rather than a logged-out one.
+  * Everything else is a rebuildable cache. The SQLite catalogue is a mirror of
+    the Jellyfin server (library metadata, watch history, offline downloads);
+    signing in again reproduces it, and watch state lives on the server anyway.
+    Backing it up would export a user's library and viewing history to their
+    Google account for no gain.
+
+  Exclude rules are listed per domain rather than relying on "root" alone,
+  because database/, shared_prefs/, files/ and external storage are addressed
+  as their own domains by the extraction engine.
+-->
+<data-extraction-rules>
+    <cloud-backup>
+        <exclude domain="root" />
+        <exclude domain="file" />
+        <exclude domain="database" />
+        <exclude domain="sharedpref" />
+        <exclude domain="external" />
+    </cloud-backup>
+    <device-transfer>
+        <exclude domain="root" />
+        <exclude domain="file" />
+        <exclude domain="database" />
+        <exclude domain="sharedpref" />
+        <exclude domain="external" />
+    </device-transfer>
+</data-extraction-rules>