App: restart the server by itself when it stops, and a Try Again button on the error page

A server that had been up for a minute starts again without asking. One that
stops sooner shows the error page, now headed "Frame Control stopped", with a
Try Again button (the menu item was the only way, and hard to find).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/preload.js

@@ -13,6 +13,7 @@ contextBridge.exposeInMainWorld("frameApp", {
   notify: (message, request) => ipcRenderer.invoke("comfort:notify", message, request),
   readClipboard: () => ipcRenderer.invoke("clipboard:read"),
   setUpConnection: () => ipcRenderer.invoke("connection:setup"),
+  restartServer: () => ipcRenderer.invoke("server:restart"),  // the "couldn't start" page's Try Again
   // The Frame menu's headset switcher: the page tells it the headsets, and hears picks.
   devicesChanged: (list) => ipcRenderer.send("devices:changed", list),
   onUseDevice: (cb) => {