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/__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')
+  })
+})