feat(dashboard): implement dashboard data structure (Task 1.3)

Add dashboard configuration to extensionSettings and create default layout system:

State Management (state.js):
- Added extensionSettings.dashboard with version 2
- gridConfig: columns (12), rowHeight (80px), gap (12px), snapToGrid, showGrid
- tabs: Array of tab objects with widgets
- defaultTab: ID of tab to show on load
- Comprehensive inline documentation of structure

Default Layout Generator (defaultLayout.js):
- generateDefaultDashboard() - Creates 2-tab default layout
  - "Status" tab: userStats, infoBox, presentCharacters (3 widgets)
  - "Inventory" tab: inventory widget (1 widget)
- migrateV1ToV2Dashboard() - Migrates v1.x settings to v2.0
  - Respects user's visibility preferences (showUserStats, etc.)
  - Removes hidden widgets from migrated layout
  - Preserves user data during migration
- validateDashboardConfig() - Validates dashboard structure
- Utility functions: getWidgetCount(), findWidget()

Persistence Layer (persistence.js):
- Auto-migration on loadSettings() for existing users
- Validates dashboard config on load
- Regenerates default if config invalid or missing
- Seamless backward compatibility

Test Suite (defaultLayout.test.html):
- 4 test scenarios with visual verification
- Tests generation, validation, migration, utilities
- Live dashboard JSON preview
- Statistics panel (version, tabs, widgets, grid config)

Features:
- Automatic migration from v1.x hardcoded panel
- Preserves user preferences during migration
- Validates all dashboard configs on load
- Generates sensible defaults for new users

Acceptance Criteria Met:
✓ Dashboard config persists in extensionSettings
✓ Default layout generates on first load
✓ Existing users see migrated layout preserving their preferences
✓ All data structures validated

Epic 1, Task 1.3 Complete (1-2 day estimate, <10 min actual)

src/core/state.js

@@ -77,7 +77,41 @@ export let extensionSettings = {
         stored: 'list',   // 'list' or 'grid' view mode for Stored section
         assets: 'list'    // 'list' or 'grid' view mode for Assets section
     },
-    debugMode: false // Enable debug logging visible in UI (for mobile debugging)
+    debugMode: false, // Enable debug logging visible in UI (for mobile debugging)
+
+    // Dashboard v2.0 Configuration
+    dashboard: {
+        version: 2, // Dashboard config version
+
+        gridConfig: {
+            columns: 12,           // Grid columns
+            rowHeight: 80,         // Pixels per row
+            gap: 12,               // Gap between widgets (px)
+            snapToGrid: true,      // Auto-snap enabled
+            showGrid: true         // Show grid lines in edit mode
+        },
+
+        tabs: [
+            // Default tabs will be generated by generateDefaultDashboard()
+            // Structure:
+            // {
+            //   id: 'tab-status',
+            //   name: 'Status',
+            //   icon: '📊',
+            //   order: 0,
+            //   widgets: [
+            //     {
+            //       id: 'widget-1',
+            //       type: 'userStats',
+            //       x: 0, y: 0, w: 6, h: 3,
+            //       config: {}
+            //     }
+            //   ]
+            // }
+        ],
+
+        defaultTab: 'tab-status' // Which tab to show on load
+    }
 };
 
 /**