One updater for app and server: updater in core; watch-together-server --upgrade / --check-update; update.sh uses the server

The server's version is stamped like the app's (APP_VERSION) when packaged,
so it can compare against the manifest; --upgrade swaps the whole install
folder (app and server) and asks for a restart. update.sh/.bat no longer
need GTK.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/server/bin/server.dart

@@ -30,6 +30,11 @@ Future<void> main(List<String> args) async {
         help: 'cookies.txt or browser-extension JSON export to log that yt-dlp in (sites that refuse guests). '
             'An account login: keep it private. Copied into --yt-dlp-dir at start.')
     ..addOption('config', help: 'YAML or JSON config file (keys as in AppConfig); overrides the other flags.')
+    ..addFlag('upgrade',
+        negatable: false,
+        help: 'Update this install (the whole folder, app included) to the latest release, then exit. '
+            'Restart the server afterwards to run it.')
+    ..addFlag('check-update', negatable: false, help: 'Print whether a newer release exists, then exit.')
     ..addFlag('help', abbr: 'h', negatable: false, help: 'Show this help.');
   final ArgResults parsed;
   try {
@@ -41,7 +46,14 @@ Future<void> main(List<String> args) async {
   }
   if (parsed['help'] as bool) {
     // ignore: avoid_print
-    print('watch-together-server (proto ${RelayConstants.protoVersion})\n\n${parser.usage}');
+    print('watch-together-server $appVersion (proto ${RelayConstants.protoVersion})\n\n${parser.usage}');
+    return;
+  }
+  if (parsed['upgrade'] as bool || parsed['check-update'] as bool) {
+    exitCode = await runUpgrade(
+        program: 'watch-together-server',
+        checkOnly: !(parsed['upgrade'] as bool),
+        restartNote: (v) => 'updated to $v — restart the server to run $v.');
     return;
   }
 

packages/server/pubspec.lock

@@ -17,6 +17,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "14.3.0"
+  archive:
+    dependency: transitive
+    description:
+      name: archive
+      sha256: "6c5bcd986e06b94e3c40244af471750840a3d2341d1f9763a1100a14add517b4"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.3.0"
   args:
     dependency: "direct main"
     description:
@@ -81,6 +89,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.7"
+  ffi:
+    dependency: transitive
+    description:
+      name: ffi
+      sha256: "6d7fd89431262d8f3125e81b50d3847a091d846eafcd4fdb88dd06f36d705a45"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.2.0"
   file:
     dependency: transitive
     description:
@@ -201,6 +217,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.5.3"
+  posix:
+    dependency: transitive
+    description:
+      name: posix
+      sha256: bc1bad54ad2b735816e31f8d4600cfde6c7839975085ddfbca48b6c9f7c4044e
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.5.2"
   pub_semver:
     dependency: transitive
     description:
@@ -319,7 +343,7 @@ packages:
       path: "../core"
       relative: true
     source: path
-    version: "0.1.0"
+    version: "1.0.1"
   term_glyph:
     dependency: transitive
     description: