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/styles/layout.css

@@ -40,6 +40,23 @@
     "player   player   player";
 }
 
+/* Resize grips — replace the native GTK grips lost when decorations: false */
+.app-shell[data-titlebar]::before,
+.app-shell[data-titlebar]::after {
+  content: '';
+  position: fixed;
+  bottom: 0;
+  width: 14px;
+  height: 14px;
+  pointer-events: none;
+  background-image: radial-gradient(circle, var(--text-muted) 1.5px, transparent 1.5px);
+  background-size: 4px 4px;
+  opacity: 0.35;
+  z-index: 9999;
+}
+.app-shell[data-titlebar]::before { left: 0; }
+.app-shell[data-titlebar]::after  { right: 0; }
+
 .titlebar {
   grid-area: titlebar;
   display: flex;
@@ -1563,6 +1580,15 @@
   max-width: 100%;
 }
 
+.app-shell[data-mobile][data-titlebar] {
+  grid-template-rows: var(--titlebar-height) 1fr auto auto;
+  grid-template-areas:
+    "titlebar"
+    "main"
+    "bottomnav"
+    "player";
+}
+
 /* Hide the sidebar collapse button on mobile */
 .app-shell[data-mobile] .collapse-btn {
   display: none;