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

contrib/plugins/calendar/main.lua

@@ -143,9 +143,13 @@ end
 --------------------------------------------------------------------------------
 
 -- Get events within a date range (inclusive)
-function M.get_events(params)
+function M.get_events(params, end_date)
+  -- Backward compat: support positional (start, end) and table {start=, end=}
+  if type(params) == "string" then
+    return M.get_events{ start_date = params, ["end"] = end_date or params }
+  end
   local start_date = params.start_date or params.start
-  local end_date = params.end_date or params["end"] or params.end_date
+  local end_date = params["end"] or params.end_date or params.end_date
   if not start_date then error("start_date required") end
   if not end_date then end_date = start_date end
   return verstak.db.query(
@@ -244,7 +248,15 @@ function M.create_event(opts)
 end
 
 -- Update an event (partial fields)
-function M.update_event(params)
+function M.update_event(params, fields)
+  -- Backward compat: support positional (id, fields) and table { id = ..., ... }
+  if type(params) == "string" then
+    local t = { id = params }
+    if fields then
+      for k, v in pairs(fields) do t[k] = v end
+    end
+    return M.update_event(t)
+  end
   local id = params.id
   if not id then error("event id required") end
   local old = verstak.db.query_row("SELECT * FROM events WHERE id = ?", id)