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/inventoryWidget.js

@@ -135,14 +135,17 @@ export function registerInventoryWidget(registry, dependencies) {
     function renderSubTabs(activeTab) {
         return `
             <div class="rpg-inventory-subtabs">
-                <button class="rpg-inventory-subtab ${activeTab === 'onPerson' ? 'active' : ''}" data-tab="onPerson">
-                    On Person
+                <button class="rpg-inventory-subtab ${activeTab === 'onPerson' ? 'active' : ''}" data-tab="onPerson" title="On Person">
+                    <i class="fa-solid fa-user"></i>
+                    <span class="rpg-subtab-label">On Person</span>
                 </button>
-                <button class="rpg-inventory-subtab ${activeTab === 'stored' ? 'active' : ''}" data-tab="stored">
-                    Stored
+                <button class="rpg-inventory-subtab ${activeTab === 'stored' ? 'active' : ''}" data-tab="stored" title="Stored">
+                    <i class="fa-solid fa-box"></i>
+                    <span class="rpg-subtab-label">Stored</span>
                 </button>
-                <button class="rpg-inventory-subtab ${activeTab === 'assets' ? 'active' : ''}" data-tab="assets">
-                    Assets
+                <button class="rpg-inventory-subtab ${activeTab === 'assets' ? 'active' : ''}" data-tab="assets" title="Assets">
+                    <i class="fa-solid fa-building"></i>
+                    <span class="rpg-subtab-label">Assets</span>
                 </button>
             </div>
         `;