Add full backup & restore to Settings (v0.14.0)

A new Backup & Restore card saves everything the app persists — songs, presets
(with their sample WAVs), themes, the autosaved project, and the settings +
input-binding DataStore blobs — into one .zip via the system file picker, and
restores from one.

Restore is defensive: it extracts to a staging folder and only swaps it into
place once the archive validates, so a corrupt/wrong file can't destroy current
data; it's zip-slip guarded and runs off the main thread. Since DataStore caches
in memory, restore prompts to reopen the app to finish applying. Covered by
BackupIoTest (round-trip replace, bad-file safety, empty-zip rejection).

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

app/build.gradle.kts

@@ -22,8 +22,8 @@ android {
         // (android.media.midi) and AAudio low-latency audio we rely on.
         minSdk = 26
         targetSdk = 34
-        versionCode = 37
-        versionName = "0.13.4"
+        versionCode = 38
+        versionName = "0.14.0"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"