feat: add Basic animation type for edges

Adds a new "basic" animation mode that uses React Flow's native animated
dashes — the entire stroke moves as dashes, with no solid line underneath.
Distinct from "snake" (moving dot) and "flow" (overlay on solid line).

- Frontend: new Basic button in EdgeModal, animMode mapping, passes
  animated={true} to BaseEdge when mode is "basic"
- Backend: normalize_animated accepts "basic" as a valid value
- Tests: EdgeModal + canvas round-trip tests for basic mode

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

@@ -122,6 +122,21 @@ describe('EdgeModal', () => {
     expect(onSubmit.mock.calls[0][0].animated).toBe('flow')
   })
 
+  it('selecting Basic sends animated: "basic"', () => {
+    const onSubmit = vi.fn()
+    render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    fireEvent.click(screen.getByText('Basic'))
+    fireEvent.click(screen.getByRole('button', { name: 'Connect' }))
+    expect(onSubmit.mock.calls[0][0].animated).toBe('basic')
+  })
+
+  it('pre-fills animation from initial "basic" string', () => {
+    const onSubmit = vi.fn()
+    render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} initial={{ animated: 'basic' }} />)
+    fireEvent.click(screen.getByRole('button', { name: 'Connect' }))
+    expect(onSubmit.mock.calls[0][0].animated).toBe('basic')
+  })
+
   it('selecting None after Snake omits animated from payload', () => {
     const onSubmit = vi.fn()
     render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} />)