feat: add Android app and end-to-end encrypted library sync

Introduce the Kotlin/Compose Android companion app and extend the TMDB
proxy into a combo server that synchronizes the whole library as an
encrypted snapshot, with all three components (Go server, Android,
desktop) sharing one zero-knowledge crypto envelope and JSON schema.

- server: add PostgreSQL-backed /sync/library (GET/PUT) with optimistic
  concurrency (revision + 409 on conflict); sync stays disabled unless
  DATABASE_URL is set. Add docker-compose with Postgres.
- desktop: add libsodium CryptoEnvelope, LibrarySerializer and SyncClient;
  storage-mode and passphrase settings; a "Sync now" toolbar action with
  conflict resolution.
- android: Room-backed library with local/cloud storage modes, encrypted
  sync client, and settings UI. The proxy server (URL + token) can be
  configured as a TMDB proxy even in local-only mode.

Crypto is identical across platforms: Argon2id (libsodium crypto_pwhash,
INTERACTIVE) + XChaCha20-Poly1305 in a versioned "UMTS" envelope, so a
snapshot encrypted on one client decrypts on the other.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

android/gradle.properties

@@ -0,0 +1,8 @@
+org.gradle.jvmargs=-Xmx2048m -Dfile.encoding=UTF-8
+org.gradle.caching=true
+# Gradle 8.11/AGP 8.7 do not support JDK 26 (the system default here), so pin
+# the build JDK to 21. Adjust if your JDK 21 path differs or you remove JDK 26.
+org.gradle.java.home=/usr/lib/jvm/java-21-openjdk
+android.useAndroidX=true
+kotlin.code.style=official
+android.nonTransitiveRClass=true