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/store/browserHandStore.ts

@@ -10,6 +10,8 @@ import Browser, {
   createSession,
   closeSession,
   listSessions,
+  screenshot as screenshotFn,
+  executeScript as executeScriptFn,
 } from '../lib/browser-client';
 import {
   BUILTIN_TEMPLATES,
@@ -247,14 +249,21 @@ export const useBrowserHandStore = create<BrowserHandState & BrowserHandActions>
       },
     });
 
-    // Create browser instance
+    // Create browser instance — reuse active session if available
     const browser = new Browser();
+    let createdOwnSession = false;
 
     try {
       store.addLog({ level: 'info', message: `开始执行模板: ${template.name}` });
 
-      // Start browser session
-      await browser.start({ headless: true });
+      // Attach to existing session or start a new one
+      if (store.activeSessionId) {
+        browser.connect(store.activeSessionId);
+        createdOwnSession = false;
+      } else {
+        await browser.start({ headless: true });
+        createdOwnSession = true;
+      }
 
       // Create execution context
       const context = {
@@ -322,7 +331,10 @@ export const useBrowserHandStore = create<BrowserHandState & BrowserHandActions>
 
       throw error;
     } finally {
-      await browser.close();
+      // Only close the session if we created it (no pre-existing active session)
+      if (createdOwnSession) {
+        await browser.close();
+      }
     }
   },
 
@@ -340,10 +352,8 @@ export const useBrowserHandStore = create<BrowserHandState & BrowserHandActions>
     });
 
     try {
-      const browser = new Browser();
-      await browser.start();
-
-      const result = await browser.eval(script, args);
+      // Use the standalone function with the existing session — no new session created
+      const result = await executeScriptFn(store.activeSessionId, script, args);
 
       store.updateExecutionState({
         isRunning: false,
@@ -399,10 +409,8 @@ export const useBrowserHandStore = create<BrowserHandState & BrowserHandActions>
     }
 
     try {
-      const browser = new Browser();
-      await browser.start();
-
-      const result = await browser.screenshot();
+      // Use the standalone function with the existing session — no new session created
+      const result = await screenshotFn(store.activeSessionId);
 
       set((state) => ({
         execution: {