feat(desktop): add offline cache and verified app updates

Clean release notes in Android and desktop update dialogs. Persist downloaded audio and offline playback events per server and user; add desktop update checks, checksum verification and installer launch.

packages/i18n/locales/en/auth.json

@@ -51,9 +51,9 @@
     "copy_server": "Copy the server address",
     "step_updates": {
       "android": "Sign in with your account. The app offers new versions by itself.",
-      "windows": "Sign in with your account. Get a new version from this page.",
-      "linux": "Sign in with your account. Get a new version from this page.",
-      "macos": "Sign in with your account. Get a new version from this page."
+      "windows": "Sign in with your account. The app will offer new versions.",
+      "linux": "Sign in with your account. The app will offer new versions through your system package installer.",
+      "macos": "Sign in with your account. The app will offer new versions."
     },
     "checksum": "SHA-256 checksum",
     "unavailable": "Apps are not available yet: release downloads are not configured on this server (docs/deployment.md).",

packages/i18n/locales/en/common.json

@@ -1,4 +1,17 @@
 {
+  "desktop": {
+    "offline": "Downloads and cache",
+    "save_track": "Download",
+    "remove_download": "Remove download",
+    "download_failed": "Could not download the track",
+    "storage_failed": "Could not open or change the cache. Check available disk space.",
+    "cache_hint": "Recent tracks are cached automatically (up to 500 MB). Manual downloads remain until removed. Available offline, including after restarting the app.",
+    "clear_recent": "Clear recent cache",
+    "offline_empty": "Played and downloaded tracks will appear here.",
+    "updates": "App updates",
+    "check_failed": "Could not check for updates. Try again later.",
+    "installer_open": "The installer is open. Complete installation and restart the app."
+  },
   "genius": {
     "description": "Lyrics, annotations and missing artwork from Genius. Local artwork takes priority.",
     "loading": "Loading…", "disabled": "Not configured", "paused": "Processing paused", "cooldown": "Waiting after Genius rate limit", "running": "Fetching data from Genius…", "queued": "Queue awaiting processing", "waiting": "Waiting for new tracks",