fix(browser): stability enhancements + MCP frontend client

S7 Browser Hand:
- Remove dead code: browser/actions.rs (314 lines of unused BrowserAction/ActionResult types)
- Fix browser_scrape_page: log failed selector matches instead of silently swallowing errors
- Fix element_to_info: document known limitation for always-None location/size fields
- Fix browserHandStore: reuse activeSessionId in executeScript/takeScreenshot/executeTemplate
  instead of creating orphan Browser sessions
- Add Browser.connect(sessionId) method for session reuse

MCP Frontend:
- Add desktop/src/lib/mcp-client.ts (77 lines) — typed client for MCP Tauri commands
  (startMcpService, stopMcpService, listMcpServices, callMcpTool)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/lib/browser-client.ts

@@ -324,6 +324,14 @@ export class Browser {
     return this.sessionId;
   }
 
+  /**
+   * Attach to an existing session by ID.
+   * Use this to reuse a session created elsewhere without spawning a new one.
+   */
+  connect(sessionId: string): void {
+    this.sessionId = sessionId;
+  }
+
   /**
    * Close browser session
    */