chore: remove computer use (re-applied after main merge)

The merge of main into this branch (0907d87) hit a modify/delete conflict
for the computer-use module — main removed it in 6761f31 while this branch
had extended it — and the conflict was resolved by keeping the branch's
copies, silently resurrecting the whole feature.

This re-applies 6761f31's removal on top of the browser-use work:
- deletes the computer-use server module, client panel, settings tab,
  desktop-agent websocket service, electron computerAgent, and the
  computer-semantics build scripts
- strips computer-use wiring from shared files (server/index.js, electron
  launcher/main, MainContent, settings, i18n) while preserving browser-use
- removes src/constants/featureFlags.ts (existed only for the CU menu flag)

browser-use (camoufox/noVNC) is untouched. typecheck passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

electron/desktopWindow.js

@@ -4,10 +4,6 @@ import { ViewHost } from './viewHost.js';
 
 const TITLEBAR_HEIGHT = 44;
 const AUTH_TOKEN_STORAGE_KEY = 'auth-token';
-// TODO: Re-enable Computer Use menus after fixing the MCP server connection
-// between the desktop app and the web UI.
-const COMPUTER_USE_MENUS_ENABLED = false;
-
 function isAllowedPermissionOrigin(sourceUrl, controlPlaneUrl) {
   try {
     const source = new URL(sourceUrl);
@@ -437,17 +433,6 @@ export class DesktopWindowManager {
             accelerator: 'CmdOrCtrl+Shift+E',
             click: () => void this.actions.showEnvironmentPicker().catch((error) => this.actions.showError('Could not switch environment', error)),
           },
-          { type: 'separator' },
-          {
-            label: 'Services',
-            visible: COMPUTER_USE_MENUS_ENABLED,
-            submenu: [
-              {
-                label: 'Computer Use',
-                click: () => void this.showDesktopSettings(),
-              },
-            ],
-          },
           {
             label: 'Diagnostics',
             submenu: [