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>

build-windows.sh

@@ -11,6 +11,13 @@
 #   dist/                          portable build (zip & ship this folder)
 #   UltimateMediaTracker-Setup.exe NSIS installer (if makensis is installed)
 #
+# Prerequisites (MSYS2 MINGW64):
+#   pacman -S mingw-w64-x86_64-qt6 mingw-w64-x86_64-qt6-tools \
+#             mingw-w64-x86_64-libsodium mingw-w64-x86_64-cmake \
+#             mingw-w64-x86_64-ninja mingw-w64-x86_64-pkgconf
+# libsodium provides the crypto for the encrypted library sync; its DLL is
+# picked up automatically by the ldd bundling loop below.
+#
 set -euo pipefail
 cd "$(dirname "$0")"