fix(plugins): JSON-serialize CallFunctionJSON return values + backward compat Lua args

Root cause: CallFunctionJSON used .String() on Lua return values, which
for tables produces 'table: 0x...' — not valid JSON. Frontend does
JSON.parse() on the result and silently caught the parse error.

Fix:
- runtime.go: convert Lua return value to JSON via luaValueToGo +
  json.Marshal so tables become proper JSON arrays/objects
- main.lua: add backward compat in get_events() and update_event()
  to accept both positional args (start, end) and table params
- CalendarPluginPage.svelte: show errors in UI instead of silent catch;
  restructure template to always show iframe + error overlay

frontend/src/lib/CalendarPluginPage.svelte

@@ -107,6 +107,7 @@
       })
     } catch (e) {
       console.error('[CalendarPluginPage] loadCalendarData:', e)
+      error = String(e)
     }
   }
 
@@ -206,21 +207,24 @@
 <div class="plugin-page">
   {#if loading}
     <p class="loading">Загрузка…</p>
-  {:else if error}
-    <p class="error">⚠ {error}</p>
-  {:else if htmlPanel}
-    <iframe
-      bind:this={iframeEl}
-      class="plugin-frame"
-      srcdoc={htmlPanel}
-      sandbox="allow-scripts allow-same-origin"
-      title="{pluginName} panel"
-    ></iframe>
-  {:else}
+  {:else if !htmlPanel}
     <div class="empty-state">
       <p>Плагин «{pluginName}» активен, но HTML-панель не настроена.</p>
       <p class="hint">Для отображения контента добавьте поле <code>panel</code> в <code>plugin.json</code>.</p>
     </div>
+  {:else}
+    <div class="plugin-frame-container">
+      {#if error}
+        <div class="plugin-error-bar">⚠ {error}</div>
+      {/if}
+      <iframe
+        bind:this={iframeEl}
+        class="plugin-frame"
+        srcdoc={htmlPanel}
+        sandbox="allow-scripts allow-same-origin"
+        title="{pluginName} panel"
+      ></iframe>
+    </div>
   {/if}
 </div>
 
@@ -232,5 +236,17 @@
   .error { color: #f87171; }
   .empty-state { text-align: center; padding: 3rem 1rem; color: var(--text-dim, #888); }
   .empty-state code { background: var(--surface-alt, #252538); padding: 0.15rem 0.4rem; border-radius: 3px; font-size: 0.85rem; }
+  .plugin-frame-container { flex: 1; display: flex; flex-direction: column; min-height: 0; }
+  .plugin-error-bar {
+    padding: 0.5rem 1rem;
+    background: rgba(248, 113, 113, 0.12);
+    border: 1px solid #f87171;
+    border-bottom: none;
+    border-radius: 8px 8px 0 0;
+    color: #f87171;
+    font-size: 0.85rem;
+    flex-shrink: 0;
+  }
   .plugin-frame { flex: 1; border: 1px solid var(--border, #2a2a3e); border-radius: 8px; background: #fff; width: 100%; min-height: 400px; display: block; }
+  .plugin-frame-container .plugin-frame { border-radius: 0 0 8px 8px; border-top: none; }
 </style>