fix(zone-modal): center slider thumb on track, drop dead code, cover a11y

- Center webkit slider thumb on 4px track (margin-top: -4.5px) so the
  opacity bullet no longer sits below the bar.
- Drop redundant inline accentColor; CSS module owns track color.
- Remove dead ::-ms-* prefix rules.
- Tests: assert aria-labels on grid/select triggers, cover getFontLabel
  happy path + unknown-value fallback via the Font selector DOM.

frontend/src/components/modals/GroupRectModal.module.css

@@ -1,4 +1,15 @@
-/* Custom colored slider track for accent color */
+/* Slider container: strip native chrome so custom track/thumb align cleanly */
+.slider-thumb {
+  -webkit-appearance: none;
+  appearance: none;
+  background: transparent;
+  height: 13px; /* match thumb height so vertical centering is the input's box center */
+}
+.slider-thumb:focus {
+  outline: none;
+}
+
+/* Track */
 .slider-accent::-webkit-slider-runnable-track {
   height: 4px;
   background: #00d4ff;
@@ -9,23 +20,17 @@
   background: #00d4ff;
   border-radius: 2px;
 }
-.slider-accent::-ms-fill-lower,
-.slider-accent::-ms-fill-upper {
-  background: #00d4ff;
-  border-radius: 2px;
-}
 
-/* Ensure the slider bar is visible on dark backgrounds */
-.slider-thumb {
-  background: transparent;
-}
-/* Custom slider thumb for smaller circle */
+/* Thumb — must offset on webkit so it centers on the 4px track */
 .slider-thumb::-webkit-slider-thumb {
+  -webkit-appearance: none;
+  appearance: none;
   width: 13px;
   height: 13px;
   border-radius: 50%;
   background: #00d4ff;
   border: 2px solid #21262d;
+  margin-top: -4.5px; /* (13 - 4) / 2 */
 }
 .slider-thumb::-moz-range-thumb {
   width: 13px;
@@ -34,10 +39,3 @@
   background: #00d4ff;
   border: 2px solid #21262d;
 }
-.slider-thumb::-ms-thumb {
-  width: 13px;
-  height: 13px;
-  border-radius: 50%;
-  background: #00d4ff;
-  border: 2px solid #21262d;
-}

frontend/src/components/modals/GroupRectModal.tsx

@@ -235,9 +235,8 @@ export function GroupRectModal({ open, onClose, onSubmit, onDelete, initial, tit
                       max={100}
                       value={alpha}
                       onChange={(e) => set(key, rgbaToHex8(hex6, Number(e.target.value)))}
-                      className={`w-full h-1 cursor-pointer mt-2 ${styles['slider-thumb']} ${styles['slider-accent']}`}
+                      className={`w-full cursor-pointer mt-2 ${styles['slider-thumb']} ${styles['slider-accent']}`}
                       title={`Opacity: ${alpha}%`}
-                      style={{ accentColor: '#00d4ff' }}
                     />
                     <span className="text-[9px] text-muted-foreground/60">{label} {alpha}%</span>
                   </div>

frontend/src/components/modals/__tests__/GroupRectModal.test.tsx

@@ -37,6 +37,16 @@ describe('GroupRectModal', () => {
     expect(submitted.z_order).toBe(1)
   })
 
+  it('exposes aria-labels on grid buttons and select triggers', () => {
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} />)
+    expect(screen.getByLabelText('Font selector')).toBeDefined()
+    expect(screen.getByLabelText('Z-order selector')).toBeDefined()
+    expect(screen.getByLabelText('Text position ↘')).toBeDefined()
+    expect(screen.getByLabelText('Label position Inside')).toBeDefined()
+    expect(screen.getByLabelText('Border style Solid')).toBeDefined()
+    expect(screen.getByLabelText('Border width 1px')).toBeDefined()
+  })
+
   it('calls onClose when Cancel is clicked', () => {
     const onClose = vi.fn()
     render(<GroupRectModal open onClose={onClose} onSubmit={vi.fn()} />)
@@ -308,3 +318,24 @@ describe('GroupRectModal', () => {
     expect(screen.getByText(/Background 5%/)).toBeInTheDocument()
   })
 })
+
+describe('GroupRectModal font label rendering', () => {
+  it('renders the human font label in the Select trigger (default inter)', () => {
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} />)
+    const trigger = screen.getByLabelText('Font selector')
+    expect(trigger.textContent).toContain('Inter (sans-serif)')
+  })
+
+  it('falls back to raw value when font is unknown', () => {
+    render(
+      <GroupRectModal
+        open
+        onClose={vi.fn()}
+        onSubmit={vi.fn()}
+        initial={{ font: 'comic-sans-9000' }}
+      />
+    )
+    const trigger = screen.getByLabelText('Font selector')
+    expect(trigger.textContent).toContain('comic-sans-9000')
+  })
+})