Implement client-side file scanning via Electron IPC with gitignore

- Add fs:scanDirectory IPC handler in main process
- Scan workspace recursively with proper gitignore support
- Use 'ignore' library for accurate gitignore pattern matching
- Expose scanDirectory via electronAPI in preload
- Call IPC from renderer instead of direct fs access
- Cache scanned files for fast filtering
- Scroll to top and highlight first item on results update
- Always exclude .git and node_modules directories

package.json

@@ -19,6 +19,7 @@
     "@opencode-ai/sdk": "0.15.13",
     "@solidjs/router": "^0.13.0",
     "electron": "38.4.0",
+    "ignore": "7.0.5",
     "lucide-solid": "^0.300.0",
     "marked": "^12.0.0",
     "shiki": "^3.13.0",