feat: enable Computer Use on Windows and Linux (#145)

Remove macOS-only guards so Computer Use works cross-platform:
- main.tsx: allow CHICAGO_MCP on any known platform (not just macos)
- swiftLoader.ts: remove darwin-only throw, let the backend handle it
- computer-use-input: dispatch to darwin/win32/linux backends
- computer-use-swift: rename loadDarwin→loadBackend, dispatch all platforms

Co-authored-by: yi7503 <yi7503@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.tsx

@@ -2434,7 +2434,7 @@ async function run(): Promise<CommanderCommand> {
       // shipped without incident; chicago places itself correctly.
       if (
         feature('CHICAGO_MCP') &&
-        getPlatform() === 'macos' &&
+        getPlatform() !== 'unknown' &&
         !getIsNonInteractiveSession()
       ) {
         try {