fix(dashboard): fix resize, drag-drop, overflow, and add auto-migration

Multiple critical fixes for dashboard v2:

**1. ResizeHandler error - updateContainerWidth is not a function**
- resizeHandler.js:288 was calling non-existent method
- Removed call - containerWidth tracked by ResizeObserver
- Resizing now functional

**2. DragDrop bug - widgets can't be released**
- endDrag() destructured widgets, originalX, originalY from dragState
- These fields were never added in startDrag()
- Added widgets parameter to initWidget() and startDrag()
- Store originalX, originalY, widgets in dragState
- dashboardManager now passes current tab widgets
- Widgets can now be dropped properly

**3. Widget content overflow**
- Added base .rpg-widget CSS: overflow: hidden, box-sizing: border-box
- Prevents content extending beyond widget bounds
- max-width: 100% on children

**4. Automatic layout migration**
- Old 12-column layouts (w: 8, w: 12) cause 500%+ widths in 2-4 column grid
- Added migrateOldLayouts() method
- Detects widgets with w > current column count
- Runs auto-layout to reposition for responsive grid
- Clears and re-renders current tab with new positions
- Saves migrated layout automatically

**5. Tab rendering**
- Implemented renderTabs() method
- Displays tab buttons with icons and names
- Active state highlighting
- Click handlers to switch tabs

**6. Collision prevention**
- Modified dragDrop endDrag() to check collisions
- Same-size widgets: swap positions
- Different sizes: revert to original
- Prevents overlapping widgets

**7. Edit mode fixes**
- Fixed edit button to call toggleEditMode()
- Added CSS to hide resize handles when not in edit mode
- Handles only visible in edit mode

**8. Icon-only header buttons**
- Auto-Arrange and Edit buttons now icon-only
- Saves horizontal space in header

All issues from user testing resolved.

src/systems/dashboard/resizeHandler.js

@@ -284,8 +284,7 @@ export class ResizeHandler {
         const deltaX = clientX - startX;
         const deltaY = clientY - startY;
 
-        // Get column/row size in pixels
-        this.gridEngine.updateContainerWidth();
+        // Get column/row size in pixels (containerWidth already set by ResizeObserver in DashboardManager)
         const totalGaps = this.gridEngine.gap * (this.gridEngine.columns + 1);
         const colWidth = (this.gridEngine.containerWidth - totalGaps) / this.gridEngine.columns;
         const rowHeight = this.gridEngine.rowHeight;