feat: add view mode toggle with compact/cozy/comfortable options

Implemented message density control with three view modes:
- Compact: Tight spacing (10px gaps), smaller text (13px), small avatars (24px)
- Cozy: Balanced spacing (16px gaps), default text (14px) - default mode
- Comfortable: Spacious layout (24px gaps), larger text (15px), larger avatars (32px)

Features:
- View mode selector in Appearance tab
- Persistent preference saved to localStorage
- Dynamic CSS classes applied to body element
- Adjusts message padding, gaps, font sizes, avatar sizes
- Scales headings, code blocks, and all UI elements proportionally

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/index.html

@@ -359,6 +359,16 @@
               <small style="color: var(--text-secondary); margin-top: 4px; display: block;">Choose your preferred color scheme</small>
             </div>
 
+            <div class="form-group">
+              <label for="view-mode-select">View Mode</label>
+              <select id="view-mode-select" class="view-mode-select">
+                <option value="compact">Compact - Tight spacing, smaller text</option>
+                <option value="cozy">Cozy - Balanced spacing (Default)</option>
+                <option value="comfortable">Comfortable - Spacious layout</option>
+              </select>
+              <small style="color: var(--text-secondary); margin-top: 4px; display: block;">Adjust message density and spacing</small>
+            </div>
+
             <div class="theme-preview-container">
               <div class="theme-preview-label">Preview</div>
               <div class="theme-preview">