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/LiveView.tsx

@@ -68,10 +68,14 @@ function LiveViewCanvas() {
     }
 
     // Already handled synchronously in useState initializer
-    const key = new URLSearchParams(window.location.search).get('key')
+    const search = new URLSearchParams(window.location.search)
+    const key = search.get('key')
     if (!key) return
+    // Optional ?design=<id> selects which canvas to render; backend falls back
+    // to the first design when omitted.
+    const design = search.get('design') ?? undefined
 
-    liveviewApi.load(key)
+    liveviewApi.load(key, design)
       .then((res) => {
         const { nodes: apiNodes, edges: apiEdges } = res.data
         const proxmoxMap = new Map<string, boolean>(