feat(calendar): add feed subscription, bulk export, share & Google Cal

- /api/kalender.ics SSR route: ICS feed of all calendar items (webcal-subscribable, Cache 1h)
- buildICSFeed() / downloadICSFeed() for multi-event ICS export
- googleCalUrl() deep-link pre-fills Google Calendar create form
- Web Share API per event (navigator.share on mobile, clipboard fallback on desktop)
  — "Kopiert\!" toast per event for 2s after clipboard write
- Actions bar (subscribe + bulk export) only rendered after onMount
  — Apple platforms: webcal:// link opens Calendar.app natively
  — All others: direct ICS download via <a download>

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

src/lib/calendar-ics.ts

@@ -122,6 +122,72 @@ export function downloadICS(item: CalendarItemICS, filename: string): void {
   URL.revokeObjectURL(url);
 }
 
+/** Builds a VCALENDAR with multiple VEVENTs — for feed/bulk export. */
+export function buildICSFeed(items: CalendarItemICS[], calName = "Windwiderstand Termine"): string {
+  const now = new Date();
+  const lines: string[] = [
+    "BEGIN:VCALENDAR",
+    "VERSION:2.0",
+    "PRODID:-//windwiderstand.de//calendar//DE",
+    "CALSCALE:GREGORIAN",
+    "METHOD:PUBLISH",
+    foldLine(`X-WR-CALNAME:${escapeText(calName)}`),
+    "X-WR-TIMEZONE:Europe/Berlin",
+    "REFRESH-INTERVAL;VALUE=DURATION:PT1H",
+    "X-PUBLISHED-TTL:PT1H",
+  ];
+  for (const item of items) {
+    const start = item.start;
+    const end = item.end ?? new Date(start.getTime() + 60 * 60 * 1000);
+    const uid = item.uid ?? deterministicUid(item.title, start);
+    lines.push(
+      "BEGIN:VEVENT",
+      `UID:${uid}`,
+      `DTSTAMP:${fmtICSDate(now)}`,
+      `DTSTART:${fmtICSDate(start)}`,
+      `DTEND:${fmtICSDate(end)}`,
+      foldLine(`SUMMARY:${escapeText(item.title)}`),
+    );
+    if (item.description) lines.push(foldLine(`DESCRIPTION:${escapeText(item.description)}`));
+    if (item.location) lines.push(foldLine(`LOCATION:${escapeText(item.location)}`));
+    if (item.url) lines.push(foldLine(`URL:${item.url}`));
+    lines.push("END:VEVENT");
+  }
+  lines.push("END:VCALENDAR");
+  return lines.join("\r\n");
+}
+
+/** Triggers a browser download of a multi-event ICS file. */
+export function downloadICSFeed(items: CalendarItemICS[], filename: string): void {
+  if (typeof window === "undefined") return;
+  const safeName = filename.replace(/[/\\:*?"<>|]/g, "_") || "termine.ics";
+  const ics = buildICSFeed(items);
+  const blob = new Blob([ics], { type: "text/calendar;charset=utf-8" });
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement("a");
+  a.href = url;
+  a.download = safeName.endsWith(".ics") ? safeName : `${safeName}.ics`;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  URL.revokeObjectURL(url);
+}
+
+/** Deep-link to Google Calendar's pre-filled create form. */
+export function googleCalUrl(item: CalendarItemICS): string {
+  const fmt = (d: Date) =>
+    d.toISOString().replace(/[-:]/g, "").replace(/\.\d+Z$/, "Z");
+  const end = item.end ?? new Date(item.start.getTime() + 60 * 60 * 1000);
+  const params = new URLSearchParams({
+    action: "TEMPLATE",
+    text: item.title,
+    dates: `${fmt(item.start)}/${fmt(end)}`,
+  });
+  if (item.description) params.set("details", item.description);
+  if (item.location) params.set("location", item.location);
+  return `https://calendar.google.com/calendar/render?${params}`;
+}
+
 /** OpenStreetMap search URL for an address or place name. */
 export function mapsUrl(location: string): string {
   return `https://www.openstreetmap.org/search?query=${encodeURIComponent(location)}`;