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/widgets/presentCharactersWidget.js

@@ -237,7 +237,7 @@ export function registerPresentCharactersWidget(registry, dependencies) {
         description: 'Character cards with avatars, traits, and relationships',
         category: 'scene',
         minSize: { w: 2, h: 2 },
-        defaultSize: { w: 2, h: 3 },
+        defaultSize: { w: 2, h: 2 }, // Compact size fits both mobile and desktop viewports
         maxAutoSize: { w: 4, h: 5 }, // Max size for auto-arrange expansion (supports up to 4-col on large displays)
         requiresSchema: false,