feat(chat): write API keys to OpenClaw and embed Control UI for chat

Part 1: API Key Integration
- Create electron/utils/openclaw-auth.ts to write keys to
  ~/.openclaw/agents/main/agent/auth-profiles.json
- Update provider:save and provider:setApiKey IPC handlers to
  persist keys to OpenClaw auth-profiles alongside ClawX storage
- Save API key to OpenClaw on successful validation in Setup wizard
- Pass provider API keys as environment variables when starting
  the Gateway process (ANTHROPIC_API_KEY, OPENROUTER_API_KEY, etc.)

Part 2: Embed OpenClaw Control UI for Chat
- Replace custom Chat UI with <webview> embedding the Gateway's
  built-in Control UI at http://127.0.0.1:{port}/?token={token}
- Add gateway:getControlUiUrl IPC handler to provide tokenized URL
- Enable webviewTag in Electron BrowserWindow preferences
- Override X-Frame-Options/CSP headers to allow webview embedding
- Suppress noisy control-ui token_mismatch stderr messages
- Add loading/error states for the embedded webview

This fixes the "No API key found for provider" error and replaces
the buggy custom chat implementation with OpenClaw's battle-tested
Control UI.

electron/preload/index.ts

@@ -22,6 +22,7 @@ const electronAPI = {
         'gateway:restart',
         'gateway:rpc',
         'gateway:health',
+        'gateway:getControlUiUrl',
         // OpenClaw
         'openclaw:status',
         'openclaw:isReady',