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/core/pubspec.lock

@@ -17,6 +17,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "14.3.0"
+  archive:
+    dependency: "direct main"
+    description:
+      name: archive
+      sha256: "6c5bcd986e06b94e3c40244af471750840a3d2341d1f9763a1100a14add517b4"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.3.0"
   args:
     dependency: transitive
     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: