feat(calendar): add drag-create, resize, recurring scope, quick-create, and duplication

- Click-drag on empty time slots to create events with pre-filled time range
- Resize events by dragging bottom edge (15-min snap, optimistic JMAP update)
- Recurring event edit/delete scope dialog (this/following/all occurrences)
- Double-click quick event creation with inline title input
- Event duplication button in modal (+1 day offset)
- Shared interaction hook for pointer-based calendar interactions

Fixes #13

lib/calendar-utils.ts

@@ -0,0 +1,54 @@
+import { parseISO } from "date-fns";
+import { parseDuration } from "@/components/calendar/event-card";
+import type { CalendarEvent } from "@/lib/jmap/types";
+
+export function getEventEndDate(event: CalendarEvent): Date {
+  const start = new Date(event.start);
+  if (!event.duration) return start;
+  return new Date(start.getTime() + parseDuration(event.duration) * 60000);
+}
+
+export function layoutOverlappingEvents(
+  events: CalendarEvent[],
+): { event: CalendarEvent; column: number; totalColumns: number }[] {
+  const sorted = [...events].sort((a, b) => {
+    const diff = new Date(a.start).getTime() - new Date(b.start).getTime();
+    if (diff !== 0) return diff;
+    return parseDuration(b.duration) - parseDuration(a.duration);
+  });
+
+  const columns: { event: CalendarEvent; end: number }[][] = [];
+  const result: { event: CalendarEvent; column: number; totalColumns: number }[] = [];
+
+  for (const event of sorted) {
+    const start = parseISO(event.start);
+    const startMin = start.getHours() * 60 + start.getMinutes();
+    const endMin = startMin + Math.max(15, parseDuration(event.duration));
+    let placed = false;
+    for (let col = 0; col < columns.length; col++) {
+      if (columns[col].every(e => e.end <= startMin)) {
+        columns[col].push({ event, end: endMin });
+        result.push({ event, column: col, totalColumns: 0 });
+        placed = true;
+        break;
+      }
+    }
+    if (!placed) {
+      columns.push([{ event, end: endMin }]);
+      result.push({ event, column: columns.length - 1, totalColumns: 0 });
+    }
+  }
+
+  const total = columns.length;
+  result.forEach(r => r.totalColumns = total);
+  return result;
+}
+
+export function formatSnapTime(minutes: number, timeFormat: "12h" | "24h"): string {
+  const h = Math.floor(minutes / 60);
+  const m = minutes % 60;
+  if (timeFormat === "12h") {
+    return `${h % 12 || 12}:${String(m).padStart(2, "0")} ${h < 12 ? "AM" : "PM"}`;
+  }
+  return `${String(h).padStart(2, "0")}:${String(m).padStart(2, "0")}`;
+}