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>

src/types/app.ts

@@ -17,7 +17,7 @@ export type ProviderModelsCacheInfo = {
   source: 'memory' | 'disk' | 'fresh';
 };
 
-export type AppTab = 'chat' | 'files' | 'shell' | 'git' | 'tasks' | 'browser' | 'computer' | `plugin:${string}`;
+export type AppTab = 'chat' | 'files' | 'shell' | 'git' | 'tasks' | 'browser' | `plugin:${string}`;
 
 export interface ProjectSession {
   id: string;