feat: add font size customization with slider control

Implemented global font size scaling with range slider:
- Adjustable from 80% to 140% in 10% increments
- Slider in Appearance tab with live value display
- Persistent preference saved to localStorage
- Updates CSS custom property --base-font-size
- Scales entire UI proportionally from 11.2px to 19.6px
- Smooth transitions with styled slider thumb
- Live preview as you drag the slider

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

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

src/index.html

@@ -369,6 +369,25 @@
               <small style="color: var(--text-secondary); margin-top: 4px; display: block;">Adjust message density and spacing</small>
             </div>
 
+            <div class="form-group">
+              <label for="font-size-slider">
+                Font Size: <span id="font-size-value">100%</span>
+              </label>
+              <input
+                type="range"
+                id="font-size-slider"
+                min="80"
+                max="140"
+                value="100"
+                step="10"
+                class="font-size-slider"
+              />
+              <div style="display: flex; justify-content: space-between; margin-top: 4px;">
+                <small style="color: var(--text-secondary);">Small (80%)</small>
+                <small style="color: var(--text-secondary);">Large (140%)</small>
+              </div>
+            </div>
+
             <div class="theme-preview-container">
               <div class="theme-preview-label">Preview</div>
               <div class="theme-preview">