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/persistence.js

@@ -18,6 +18,7 @@ import {
 } from './state.js';
 import { migrateInventory } from '../utils/migration.js';
 import { validateStoredInventory, cleanItemString } from '../utils/security.js';
+import { generateDefaultDashboard, migrateV1ToV2Dashboard, validateDashboardConfig } from '../systems/dashboard/defaultLayout.js';
 
 const extensionName = 'third-party/rpg-companion-sillytavern';
 
@@ -95,6 +96,20 @@ export function loadSettings() {
                 saveSettings(); // Persist migrated inventory
             }
         }
+
+        // Migrate to v2.0 dashboard if not present
+        if (!extensionSettings.dashboard || !extensionSettings.dashboard.tabs || extensionSettings.dashboard.tabs.length === 0) {
+            console.log('[RPG Companion] Dashboard v2.0 not found, migrating from v1.x');
+            extensionSettings.dashboard = migrateV1ToV2Dashboard(extensionSettings);
+            saveSettings(); // Persist migrated dashboard
+        } else {
+            // Validate existing dashboard config
+            if (!validateDashboardConfig(extensionSettings.dashboard)) {
+                console.warn('[RPG Companion] Dashboard config invalid, regenerating default');
+                extensionSettings.dashboard = generateDefaultDashboard();
+                saveSettings();
+            }
+        }
     } catch (error) {
         console.error('[RPG Companion] Error loading settings:', error);
         console.error('[RPG Companion] Error details:', error.message, error.stack);