fix(titlebar): window drag, resize grips and mobile-mode grid for custom titlebar

- Add core:window:allow-start-dragging capability so data-tauri-drag-region
  actually works (was silently blocked without this permission)
- Add CSS resize grips (bottom-left + bottom-right) that replace the native
  GTK grip lost when decorations: false
- Add [data-mobile][data-titlebar] grid override so the titlebar renders
  correctly in narrow-window mode instead of being hidden

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

src-tauri/capabilities/default.json

@@ -34,6 +34,7 @@
     "core:window:allow-show",
     "core:window:allow-set-fullscreen",
     "core:window:allow-is-fullscreen",
+    "core:window:allow-start-dragging",
     "core:window:allow-create",
     "core:webview:allow-create-webview-window",
     "process:allow-restart"