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/styles.css

@@ -1324,6 +1324,65 @@ body {
   cursor: pointer;
 }
 
+/* Font size slider */
+.font-size-slider {
+  width: 100%;
+  height: 6px;
+  background: var(--bg-tertiary);
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  outline: none;
+  -webkit-appearance: none;
+  appearance: none;
+  cursor: pointer;
+}
+
+.font-size-slider::-webkit-slider-thumb {
+  -webkit-appearance: none;
+  appearance: none;
+  width: 18px;
+  height: 18px;
+  background: var(--accent);
+  border-radius: 50%;
+  cursor: pointer;
+  transition: all 0.2s ease;
+  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
+}
+
+.font-size-slider::-webkit-slider-thumb:hover {
+  background: var(--accent-hover);
+  transform: scale(1.1);
+}
+
+.font-size-slider::-moz-range-thumb {
+  width: 18px;
+  height: 18px;
+  background: var(--accent);
+  border: none;
+  border-radius: 50%;
+  cursor: pointer;
+  transition: all 0.2s ease;
+  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
+}
+
+.font-size-slider::-moz-range-thumb:hover {
+  background: var(--accent-hover);
+  transform: scale(1.1);
+}
+
+.font-size-slider::-moz-range-track {
+  background: var(--bg-tertiary);
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  height: 6px;
+}
+
+#font-size-value {
+  color: var(--accent);
+  font-weight: 600;
+  font-size: 13px;
+}
+
 /* View Mode Styles */
 
 /* Compact Mode */