feat: rename Rectangle to Zone, add border width selector

- Rename "Rectangle" → "Zone" in sidebar, add modal and edit modal
- Add border width selector (1–5px, default 2px) to the Zone modal
- Border width persisted in custom_colors.border_width and applied in GroupRectNode

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

@@ -13,14 +13,15 @@ describe('GroupRectModal', () => {
   it('renders form fields when open', () => {
     render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} />)
     expect(screen.getByPlaceholderText('Zone name…')).toBeDefined()
-    expect(screen.getByText('Add Rectangle')).toBeDefined()
+    expect(screen.getByText('Add Zone')).toBeDefined()
     expect(screen.getByText('Text Position')).toBeDefined()
+    expect(screen.getByText('Border Width')).toBeDefined()
     expect(screen.getByText('Z-Order (1 = furthest back)')).toBeDefined()
   })
 
-  it('renders Edit Rectangle title when provided', () => {
-    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} title="Edit Rectangle" />)
-    expect(screen.getByText('Edit Rectangle')).toBeDefined()
+  it('renders Edit Zone title when provided', () => {
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} title="Edit Zone" />)
+    expect(screen.getByText('Edit Zone')).toBeDefined()
   })
 
   it('calls onSubmit with form data on submit', () => {
@@ -123,6 +124,46 @@ describe('GroupRectModal', () => {
     expect(submitted.border_style).toBe('dotted')
   })
 
+  it('renders Border Width section with 5 options', () => {
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} />)
+    expect(screen.getByText('Border Width')).toBeDefined()
+    expect(screen.getByText('1px')).toBeDefined()
+    expect(screen.getByText('3px')).toBeDefined()
+    expect(screen.getByText('5px')).toBeDefined()
+  })
+
+  it('defaults border_width to 2', () => {
+    const onSubmit = vi.fn()
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    fireEvent.click(screen.getByText('Add'))
+    const submitted = onSubmit.mock.calls[0][0] as GroupRectFormData
+    expect(submitted.border_width).toBe(2)
+  })
+
+  it('selects border width on click', () => {
+    const onSubmit = vi.fn()
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    fireEvent.click(screen.getByText('4px'))
+    fireEvent.click(screen.getByText('Add'))
+    const submitted = onSubmit.mock.calls[0][0] as GroupRectFormData
+    expect(submitted.border_width).toBe(4)
+  })
+
+  it('pre-fills border_width from initial prop', () => {
+    const onSubmit = vi.fn()
+    render(
+      <GroupRectModal
+        open
+        onClose={vi.fn()}
+        onSubmit={onSubmit}
+        initial={{ border_width: 5 }}
+      />
+    )
+    fireEvent.click(screen.getByText('Add'))
+    const submitted = onSubmit.mock.calls[0][0] as GroupRectFormData
+    expect(submitted.border_width).toBe(5)
+  })
+
   it('toggles border style — clicking selected style deselects back to solid', () => {
     const onSubmit = vi.fn()
     render(<GroupRectModal open onClose={vi.fn()} onSubmit={onSubmit} />)