perf: faster plugin updates, half-size first bundle, images from the chat attach on Windows

- Chat view: an attached image's path is pasted on its own after the text
  (promptForPane). claude turns a paste that is only an image path into
  [Image #n], but left a Windows path inside a paste of text as a plain path.
  Other files and @-mentions stay in the text as before.
- Plugin bootstrap: depsHash covers only what npm installs (no release
  version), so a version bump no longer reinstalls runtime deps; the client's
  installed tree is kept while its manifests are unchanged (no npm ci per
  update); the user-side build runs vite only (tsc -b cost ~25 s / ~900 MB);
  clientBuild is recorded only after a build that did not fail, so a failed
  one is retried; npm output goes to runtime/npm-install.log.
- Server: a port held by another program ends with a clear message and exit 1
  instead of an uncaught 'error' stack trace.
- Client: the onboarding wizard is lazy and rendered only before onboarding;
  usePrefersReducedMotion moved to hooks/ so PetSettings no longer pulls the
  Sagi chat/pane stack into the entry chunk (884 KB -> 431 KB, gzip 268 -> 133 KB).

server/index.js

@@ -33,6 +33,20 @@ if (process.env.GIT_EDITOR === "true") delete process.env.GIT_EDITOR;
 // server/lib/settings-env.js. Only when this
 // file IS the process: the test suite requires it dozens of times, and a
 // developer's real settings file pins DASHBOARD_DATA_DIR at their live database.
+/** What to tell the user when the dashboard cannot bind its port. The health
+ *  probe above already turned away a port held by a running dashboard, so a
+ *  port in use here belongs to some other program. */
+function listenFailure(err, port) {
+  if (err?.code === "EADDRINUSE")
+    return (
+      `Port ${port} is already in use by another program, so AgentHub cannot start there. ` +
+      `Stop that program, or run AgentHub on another port with DASHBOARD_PORT (for example DASHBOARD_PORT=4821).`
+    );
+  if (err?.code === "EACCES")
+    return `Not allowed to listen on port ${port}. Pick a port above 1024 with DASHBOARD_PORT.`;
+  return `AgentHub could not start its server: ${err?.stack || err}`;
+}
+
 if (require.main === module) {
   const settingsEnv = require("./lib/settings-env");
   settingsEnv.loadEnvFiles({
@@ -337,8 +351,12 @@ function startServer(app, port) {
   const host = resolveHost();
   const boundLoopback = isLoopbackHostname(host);
 
-  return new Promise((resolve) => {
+  return new Promise((resolve, reject) => {
+    // A port another program holds fails here, not as an uncaught 'error'
+    // event: the caller says what happened instead of a bare stack trace.
+    server.once("error", reject);
     server.listen(port, host, () => {
+      server.off("error", reject);
       // Publish the live port so the Claude Code hook handler can find this
       // server even when it bound a non-default port (the desktop app falls
       // back off 4820 when that port is already taken).
@@ -1210,12 +1228,17 @@ if (require.main === module) {
       return;
     }
     const app = createApp();
-    startServer(app, PORT).then((server) => {
-      httpServer = server;
-      startBackgroundServices();
-      // The status-area icon (server/lib/tray.js) — production servers only.
-      require("./lib/tray").startTrayWatch();
-    });
+    startServer(app, PORT)
+      .then((server) => {
+        httpServer = server;
+        startBackgroundServices();
+        // The status-area icon (server/lib/tray.js) — production servers only.
+        require("./lib/tray").startTrayWatch();
+      })
+      .catch((err) => {
+        console.error(listenFailure(err, PORT));
+        process.exit(1);
+      });
   });
 
   // Graceful shutdown — close connections and DB cleanly