Refactor search modes and optimize embedding generation

- Updated the dashboard template to hide the Code Graph Explorer feature.
- Enhanced the `executeCodexLens` function to use `exec` for better cross-platform compatibility and improved command execution.
- Changed the default `maxResults` and `limit` parameters in the smart search tool to 10 for better performance.
- Introduced a new `priority` search mode in the smart search tool, replacing the previous `parallel` mode, which now follows a fallback strategy: hybrid -> exact -> ripgrep.
- Optimized the embedding generation process in the embedding manager by batching operations and using a cached embedder instance to reduce model loading overhead.
- Implemented a thread-safe singleton pattern for the embedder to improve performance across multiple searches.

ccw/src/templates/dashboard.html

@@ -331,10 +331,12 @@
                 <i data-lucide="history" class="nav-icon"></i>
                 <span class="nav-text flex-1" data-i18n="nav.history">History</span>
               </li>
+              <!-- Hidden: Code Graph Explorer (feature disabled)
               <li class="nav-item flex items-center gap-2 mx-2 px-3 py-2.5 text-sm text-muted-foreground hover:bg-hover hover:text-foreground rounded cursor-pointer transition-colors" data-view="graph-explorer" data-tooltip="Code Graph Explorer">
                 <i data-lucide="git-branch" class="nav-icon"></i>
                 <span class="nav-text flex-1" data-i18n="nav.graphExplorer">Graph</span>
               </li>
+              -->
             </ul>
           </div>