fix: correct devcontainer workspace path mismatch

Volume was mounting ./backend to /workspaces/backend, but devcontainer.json
pointed VS Code to /workspaces/familienarchiv — causing the broken path shown
in Remote Explorer.

Now mounts the full project root to /workspaces/familienarchiv, which matches
the workspaceFolder variable. Also gives container access to frontend/ for
running npm run generate:api without leaving the devcontainer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -14,8 +14,8 @@
       "installGradle": "false"
     },
     "ghcr.io/devcontainers/features/node:1": {
-    "version": "24"
-  }
+      "version": "24"
+    }
   },
 
   // VS Code Extensions, die automatisch im Container installiert werden