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>

scripts/release/prepare-desktop-app.js

@@ -113,12 +113,6 @@ await copyRequired('electron');
 await copyRequired('dist');
 await copyRequired('public');
 
-// The desktop app still ships the standalone Computer Use desktop agent, but
-// not the full local server. Local CloudCLI is downloaded on demand.
-await copyRequired('dist-server/server/computer-use-agent.js');
-await copyIfExists('dist-server/server/computer-use-agent.js.map');
-await copyRequired('dist-server/server/modules/computer-use');
-
 const copiedRuntimeDependencies = [];
 if (await copyNodeModule('ws')) {
   copiedRuntimeDependencies.push('ws');