fix: translate the calendar sheets and label their buttons

- The custom recurrence editor and its summary were English throughout
  (labels, units, weekday and month names, "on the third Thursday");
  they now use the webmail's recurrence fragments and the app's date
  locale. The interval and occurrence counts are ICU plurals, also in
  the event details, which filled "Every {count} days" with
  `.replace`.
- The iCal subscription sheet is translated with the webmail's
  subscription strings, and its "last updated" time is relative in the
  app language. The feed errors it shows come from the catalog too.
- The calendar share sheet asked for `calendar.share.role_*` keys no
  catalog has; it now uses the webmail's sharing presets and title.
- Close, remove, refresh, unsubscribe, delete and new-task buttons get
  screen reader labels, the event editor's switches are named, color
  swatches read out their color, and task checkboxes and calendar rows
  report whether they are checked.

The calendar i18n test now covers the sheets.

src/stores/calendar-subscriptions-store.ts

@@ -15,6 +15,7 @@ import {
 import { jmapClient } from '../api/jmap-client';
 import { uploadBytes } from '../api/blob';
 import { useCalendarStore } from './calendar-store';
+import { t } from './locale-store';
 
 export interface CalendarSubscription {
   id: string;
@@ -100,13 +101,16 @@ function base64(input: string): string {
 async function fetchAndParseFeed(url: string): Promise<Partial<CalendarEvent>[]> {
   const { url: feedUrl, headers } = normalizeFeedUrl(url);
   const res = await fetch(feedUrl, { headers });
-  if (!res.ok) throw new Error(`Could not fetch feed (HTTP ${res.status})`);
+  if (!res.ok) {
+    throw new Error(t('calendar.subscription.fetch_failed', 'Could not fetch feed (HTTP {status})', { status: res.status }));
+  }
+  const tooLarge = () => new Error(t('calendar.subscription.feed_too_large', 'Feed is too large'));
   const length = Number(res.headers.get('content-length') || 0);
-  if (length > MAX_FEED_BYTES) throw new Error('Feed is too large');
+  if (length > MAX_FEED_BYTES) throw tooLarge();
   const text = await res.text();
-  if (text.length > MAX_FEED_BYTES) throw new Error('Feed is too large');
+  if (text.length > MAX_FEED_BYTES) throw tooLarge();
   if (!/BEGIN:VCALENDAR/i.test(text)) {
-    throw new Error('That URL did not return an iCalendar feed');
+    throw new Error(t('calendar.subscription.not_a_feed', 'That URL did not return an iCalendar feed'));
   }
   const bytes = new TextEncoder().encode(text);
   const { blobId } = await uploadBytes(bytes, 'text/calendar');
@@ -177,7 +181,7 @@ export const useCalendarSubscriptionsStore = create<SubscriptionsState>()(
             // best-effort
           }
           await useCalendarStore.getState().fetchCalendars();
-          throw err instanceof Error ? err : new Error('Sync failed');
+          throw err instanceof Error ? err : new Error(t('calendar.subscription.error', 'Failed to add subscription'));
         } finally {
           set({ syncing: { ...get().syncing, [sub.id]: false } });
         }
@@ -234,7 +238,7 @@ export const useCalendarSubscriptionsStore = create<SubscriptionsState>()(
             ),
           });
         } catch (err) {
-          const message = err instanceof Error ? err.message : 'Sync failed';
+          const message = err instanceof Error ? err.message : t('calendar.subscription.refresh_error', 'Failed to refresh subscription');
           set({
             subscriptions: get().subscriptions.map((s) =>
               s.id === id ? { ...s, lastError: message } : s,