fix calendar: table params for Lua functions + remove duplicate header

- CalendarPluginPage.svelte: removed <h2>pluginName — pageLabel</h2> (AppHeader already shows title)
- main.lua: all API functions now accept a single table parameter:
  - get_events(params) — reads params.start_date or params.start
  - get_event(params) — reads params.id
  - update_event(params) — reads params.id + mutable fields
  - delete_event(params) — reads params.id
  - get_expanded_events(params), get_calendar_events(params) — same
  - get_events_day(params) — reads params.date
  - create_event(opts) — already worked, no change
- Backward compatible: get_events accepts both start/start_date and end/end_date keys

frontend/src/lib/CalendarPluginPage.svelte

@@ -204,10 +204,6 @@
 </script>
 
 <div class="plugin-page">
-  <div class="plugin-page-header">
-    <h2>{pluginName} — {pageLabel}</h2>
-  </div>
-
   {#if loading}
     <p class="loading">Загрузка…</p>
   {:else if error}