fix(calendar): correct event timezone handling

- Frontend sends user_timezone (IANA, from Intl.DateTimeFormat) with
  every message POST; threaded through route → generation_task → build_context
- System prompt now tells the LLM the user's timezone so it creates
  events with the correct UTC offset (e.g. 15:00+01:00 not 15:00Z)
- Calendar tool guidance updated to require UTC offset in all event
  datetimes
- EventSlideOver: dateFromIso/timeFromIso now use JS Date to convert
  stored UTC times to local time for display; toIso includes local
  timezone offset when saving so the correct UTC time is stored

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

frontend/src/components/EventSlideOver.vue

@@ -38,17 +38,30 @@ const color = ref("");
 const projectId = ref<number | null>(null);
 
 function dateFromIso(iso: string): string {
-  return iso.slice(0, 10);
+  const d = new Date(iso);
+  if (isNaN(d.getTime())) return iso.slice(0, 10);
+  const y = d.getFullYear();
+  const m = String(d.getMonth() + 1).padStart(2, "0");
+  const day = String(d.getDate()).padStart(2, "0");
+  return `${y}-${m}-${day}`;
 }
 
 function timeFromIso(iso: string): string {
   if (!iso.includes("T")) return "09:00";
-  return iso.slice(11, 16);
+  const d = new Date(iso);
+  if (isNaN(d.getTime())) return iso.slice(11, 16);
+  return `${String(d.getHours()).padStart(2, "0")}:${String(d.getMinutes()).padStart(2, "0")}`;
 }
 
 function toIso(date: string, time: string): string {
   if (!time) return `${date}T00:00:00`;
-  return `${date}T${time}:00`;
+  // Include local timezone offset so the server stores the correct UTC time
+  const local = new Date(`${date}T${time}:00`);
+  const off = -local.getTimezoneOffset();
+  const sign = off >= 0 ? "+" : "-";
+  const h = String(Math.floor(Math.abs(off) / 60)).padStart(2, "0");
+  const min = String(Math.abs(off) % 60).padStart(2, "0");
+  return `${date}T${time}:00${sign}${h}:${min}`;
 }
 
 function resetForm() {

frontend/src/stores/chat.ts

@@ -324,6 +324,7 @@ export const useChatStore = defineStore("chat", () => {
           think,
           rag_project_id: ragProjectId ?? undefined,
           workspace_project_id: workspaceProjectId ?? undefined,
+          user_timezone: Intl.DateTimeFormat().resolvedOptions().timeZone,
         },
       );
       assistantMessageId = resp.assistant_message_id;