feat(dashboard): implement responsive header with tab scrolling and overflow menus

Add comprehensive responsive header system with Google-quality UX:

Tab Navigation:
- Add TabScrollManager for horizontal scrolling tabs
- Left/Right navigation arrows appear when scrollable
- Edge fade indicators show more content exists
- Smooth scroll behavior with momentum
- Progressive sizing: full → icon+name → icon-only
- Automatic scroll position tracking

Button Overflow System:
- Add HeaderOverflowManager with ResizeObserver
- Three responsive modes based on container width:
  * Full mode (>900px): all buttons visible
  * Overflow mode (500-900px): priority + "More" (⋮) menu
  * Compact mode (<500px): priority + hamburger (☰) menu
- Priority buttons (Lock + Edit) always visible
- Smooth transitions between modes

Dropdown Menu:
- Professional slide-down animation
- Full keyboard navigation (arrows, Home, End, Escape)
- Click-outside-to-close behavior
- ARIA attributes for accessibility
- Focus management and trap
- Auto-refresh on edit mode changes
- High z-index (10003) ensures visibility above all UI

Cross-Tab Widget Dragging:
- Add collision detection for widget placement
- Implement moveWidgetToTab() with collision avoidance
- Find available positions in target tab automatically
- Update dragDrop.js to detect tab hover
- Visual feedback with tab highlight on hover
- Proper widget positioning after cross-tab move

Additional Features:
- Sort Tab button for current-tab-only auto-layout
- Mobile optimizations with compact buttons
- Responsive breakpoints at 768px and 480px
- Hardware-accelerated animations
- Touch-friendly 44px minimum targets

Files changed:
- New: tabScrollManager.js, headerOverflowManager.js
- Modified: dashboardTemplate.html, dashboardIntegration.js
- Modified: dashboardManager.js, dragDrop.js, style.css
- Modified: editModeManager.js (lock state default)

src/systems/dashboard/editModeManager.js

@@ -28,7 +28,7 @@ export class EditModeManager {
         this.onWidgetSettings = config.onWidgetSettings;
 
         this.isEditMode = false;
-        this.isLocked = false; // Lock state prevents dragging/resizing
+        this.isLocked = true; // Start locked to prevent accidental widget moves
         this.originalLayout = null;
         this.gridOverlay = null;
         this.widgetLibrary = null;