Server on an isolate, dedicated server binary, room -> server (proto 5)

- serve(Map config, events) is the one server entrypoint: the app
  hosts it via Isolate.spawn (crash surfaced on the Host card, logs
  forwarded as [server]); bin/server.dart is the dedicated server,
  compiled into the bundle as watch-together-server. --headless gone.
- Host identity via a per-hosting hostKey in hello, replacing the
  loopback heuristic.
- Rename room -> server across code, files and wire (t:"state",
  /health name); protocol bumped to 5.
- CI compiles the server binary; isolate smoke tests in server and
  desktop suites. Docs updated.

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

.gitea/workflows/ci.yml

@@ -31,3 +31,8 @@ jobs:
         working-directory: packages/server
       - run: dart test
         working-directory: packages/server
+      # Compiling the dedicated server proves it stays Flutter-free.
+      - run: dart compile exe bin/server.dart -o build/watch-together-server
+        working-directory: packages/server
+      - run: build/watch-together-server --help
+        working-directory: packages/server