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/api/client.ts

@@ -53,7 +53,9 @@ export const edgesApi = {
 }
 
 export const liveviewApi = {
-  load: (key: string) => publicApi.get('/liveview', { params: { key } }),
+  load: (key: string, design?: string) =>
+    publicApi.get('/liveview', { params: { key, ...(design ? { design_id: design } : {}) } }),
+  getConfig: () => api.get<{ enabled: boolean; key: string | null }>('/liveview/config'),
 }
 
 export const scanApi = {