feat(liveview): header View link opens active design as read-only canvas

Add an authenticated GET /api/v1/liveview/config endpoint exposing the
configured LIVEVIEW_KEY to logged-in admins, so the UI builds a
ready-to-use share link. The header View button opens
/view?key=...&design=<activeDesignId>; LiveView forwards ?design=<id>
to the public endpoint, which renders that design's canvas.

ha-relevant: maybe

frontend/src/components/__tests__/LiveView.test.tsx

@@ -118,7 +118,15 @@ describe('LiveView (non-standalone)', () => {
     await waitFor(() => {
       expect(screen.getByTestId('react-flow')).toBeDefined()
     })
-    expect(liveviewApi.load).toHaveBeenCalledWith('correct-key')
+    expect(liveviewApi.load).toHaveBeenCalledWith('correct-key', undefined)
+  })
+
+  it('forwards ?design=<id> to the API so a specific canvas is loaded', async () => {
+    setSearch('?key=correct-key&design=elec-123')
+    vi.mocked(liveviewApi.load).mockResolvedValue(canvasPayload as never)
+    render(<LiveView />)
+    await waitFor(() => expect(screen.getByTestId('react-flow')).toBeDefined())
+    expect(liveviewApi.load).toHaveBeenCalledWith('correct-key', 'elec-123')
   })
 
   it('allows zooming out to 0.25 so large infra fits (matches the editor)', async () => {