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/dashboardTemplate.html

@@ -8,6 +8,12 @@
         </div>
 
         <div class="rpg-dashboard-header-right">
+            <!-- Auto-Layout Button -->
+            <button id="rpg-dashboard-auto-layout" class="rpg-dashboard-btn rpg-auto-layout-btn" title="Auto-Arrange Widgets">
+                <i class="fa-solid fa-table-cells-large"></i>
+                <span>Auto-Arrange</span>
+            </button>
+
             <!-- Edit Mode Toggle -->
             <button id="rpg-dashboard-edit-mode" class="rpg-dashboard-btn rpg-edit-mode-btn" title="Toggle Edit Mode">
                 <i class="fa-solid fa-pen-to-square"></i>