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/systems/dashboard/widgets/presentCharactersWidget.js

@@ -235,8 +235,8 @@ export function registerPresentCharactersWidget(registry, dependencies) {
         name: 'Present Characters',
         icon: '👥',
         description: 'Character cards with avatars, traits, and relationships',
-        minSize: { w: 4, h: 3 },
-        defaultSize: { w: 6, h: 4 },
+        minSize: { w: 2, h: 2 },
+        defaultSize: { w: 2, h: 3 },
         requiresSchema: false,
 
         render(container, config = {}) {