feat(dashboard): add auto-layout button with smart widget packing

Implements intelligent auto-layout system that efficiently arranges widgets to maximize space usage while respecting panel width constraints.

**Key Features:**
- Smart packing algorithm that sorts by widget area and finds optimal positions
- Respects responsive column count (2-4 columns based on panel width)
- Prefers full-width widgets when possible to eliminate gaps
- Fallback to narrower widths for better vertical packing
- Maintains minimum widget sizes

**Implementation:**
- GridEngine.autoLayout() - Core packing algorithm with collision detection
- DashboardManager.autoLayoutWidgets() - High-level API that re-renders after layout
- Auto-Arrange button in dashboard header (uses fa-table-cells-large icon)
- Event handler wired to call autoLayoutWidgets with preferFullWidth=true

**Algorithm Strategy:**
1. Sort widgets by area (largest first) for efficient packing
2. For each widget, try full-width placement first
3. Find first available position using row-by-row scan
4. If position is too far down, try narrower widths
5. Mark cells as occupied to prevent overlaps

**Testing Notes:**
- Works with current responsive column system (2-4 columns)
- Respects minimum sizes and column constraints
- Re-renders all widgets after repositioning
- Auto-saves layout changes

Part of Epic 2: Dashboard Widget Library

src/core/state.js

@@ -84,7 +84,9 @@ export let extensionSettings = {
         version: 2, // Dashboard config version
 
         gridConfig: {
-            columns: 12,           // Grid columns
+            // Columns calculated dynamically by GridEngine (2-4 based on panel width)
+            // Mobile (≤1000px screen): always 2 columns
+            // Desktop (>1000px screen): 2-4 columns based on panel width
             rowHeight: 80,         // Pixels per row
             gap: 12,               // Gap between widgets (px)
             snapToGrid: true,      // Auto-snap enabled