embed mode: pass embed_mode to context, add conditional body class + CSS

- board.py: pass embed_mode=True to template context when ?embed=1
- base.html: <body class="embed-mode"> when embed_mode is true
- CSS: body.embed-mode hides .sidebar, .topbar, .unified-header,
  .header-actions; removes app-layout margin/padding; adjusts editor
  padding and max-width for peek panel
- Sidebar and header now properly hidden in iframe via CSS

app/templates/base.html

@@ -88,12 +88,19 @@
         font-size: 13px;
       }
     }
+    /* Embed mode — hide sidebar + header for peek panels */
+    body.embed-mode .sidebar { display: none !important; }
+    body.embed-mode .topbar, body.embed-mode .unified-header { display: none !important; }
+    body.embed-mode .topbar-right, body.embed-mode .header-actions { display: none !important; }
+    body.embed-mode .app-layout { margin-left: 0 !important; padding-top: 0 !important; }
+    body.embed-mode .page-editor-wrapper { padding: 8px 16px !important; max-width: 100% !important; }
+    body.embed-mode .page-cover-area { padding-top: 0 !important; }
   </style>
   <script src="/static/js/htmx.min.js"></script>
   <script src="/static/js/alpine.min.js" defer></script>
   <script src="/static/js/sortable.min.js" defer></script>
 </head>
-<body hx-headers='{"X-CSRF-Token":"__CSRF_PLACEHOLDER__"}'>
+<body hx-headers='{"X-CSRF-Token":"__CSRF_PLACEHOLDER__"}'{% if embed_mode %} class="embed-mode"{% endif %}>
 <div class="app-layout" x-data="appState()">
 
   <!-- ═══════════ MOBILE SIDEBAR OVERLAY ═══════════ -->