feat(dashboard): improve mobile inventory UX and fix desktop viewport overflow

Mobile Inventory Improvements:
- Add icon-based sub-tabs (user/box/building) with responsive labels
- Desktop shows icon + label, mobile shows icon-only for compact layout
- Add proper scroll containers for inventory content with flex layout
- Increase touch drag delay from 150ms to 500ms to prevent accidental widget moves during scrolling

Widget Content Fixes:
- Add max-height constraint to .rpg-widget to prevent grid cell overflow
- Add flex properties (flex: 1, min-height: 0, overflow: auto) to all widget content
- Ensures content scrolls internally instead of expanding widget bounds
- Fix .rpg-inventory-widget to use flex properties instead of height: 100%

Layout Fixes:
- Change characters widget default size from 2x3 to 2x2 for better viewport fit
- Remove excess spacing from dashboard container (gap: 0.75rem)
- Remove vertical padding from dashboard header
- Eliminates desktop scrollbar caused by cumulative spacing

All widgets now fit properly within viewport on both desktop and mobile.

src/systems/dashboard/dragDrop.js

@@ -30,7 +30,7 @@ export class DragDropHandler {
             showCollisions: true,
             enableSnap: true,
             ghostOpacity: 0.5,
-            touchDelay: 150, // Delay before touch drag starts (ms)
+            touchDelay: 500, // Delay before touch drag starts (ms) - longer delay prevents accidental moves during scrolling
             mouseMoveThreshold: 5, // Pixels mouse must move before drag starts
             ...options
         };