Fix task pane layout at mobile - add explicit grid positioning

The task-pane was not displaying at mobile because it lacked explicit
grid positioning and width constraints. Added:
- grid-row: 2 and grid-column: 1 for explicit placement
- width: 100%, min-width: 0, max-width: 100% to ensure full width
- Moved header grid positioning into mobile breakpoint for clarity

Also updated header to have explicit grid-row: 1 positioning.

Cache bust: v=5

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

static/css/app.css

@@ -918,11 +918,22 @@ a:hover {
         grid-template-columns: 1fr;
     }
 
-    /* Header mobile optimization */
+    /* Explicit grid positioning for mobile */
     .app-header {
+        grid-row: 1;
+        grid-column: 1;
         padding: var(--space-sm) var(--space-md);
     }
 
+    /* Force task pane into the single column with full width */
+    .task-pane {
+        grid-row: 2;
+        grid-column: 1;
+        width: 100%;
+        min-width: 0;
+        max-width: 100%;
+    }
+
     .header-user span {
         display: none; /* Hide email on mobile */
     }