feat: surface per-calendar sync errors from /api/caldav/events

The server now reports per-calendar sync failures (e.g. expired
credentials) alongside an otherwise-successful events response, via an
"errors" array. Previously such failures were silently swallowed, so a
calendar could appear enabled while showing zero events with no
explanation.

- Decode the new "errors" field into SyncError (source/name/message),
  following the existing JSONSerialization + from(json:) convention.
- CalendarStore.syncErrors captures these on successful personal
  fetches (loadEvents, prefetchBackground); left untouched on a hard
  fetch failure so it isn't conflated with lastError. Group overlay
  fetches never populate it (the endpoint doesn't apply there).
- CalendarHostView shows a second red warning banner alongside the
  existing lastError banner, reusing the same visual style.

Calendarr iOS/Views/Calendar/CalendarHostView.swift

@@ -284,10 +284,14 @@ struct CalendarHostView: View {
         .accessibilityLabel(L10n.t("nav.menu", appLang))
     }
 
-    // MARK: – Error banner
+    // MARK: – Error banners
 
+    // `lastError` (whole fetch failed) and `syncErrors` (fetch succeeded, but
+    // individual calendars didn't sync) are independent conditions and can
+    // both be shown at once.
     @ViewBuilder private var errorBanner: some View {
         if let err = store.lastError { errorBannerView(err) }
+        if !store.syncErrors.isEmpty { syncErrorBannerView(store.syncErrors) }
     }
 
     private func errorBannerView(_ err: String) -> some View {
@@ -303,6 +307,23 @@ struct CalendarHostView: View {
         .background(Color.red.opacity(0.85))
     }
 
+    /// One or more calendars didn't sync on the last fetch (e.g. expired
+    /// credentials) even though they're still enabled. Same visual language
+    /// as `errorBannerView`, joined into a single compact banner.
+    private func syncErrorBannerView(_ errors: [SyncError]) -> some View {
+        let text = errors.map { "\($0.source) (\($0.name)): \($0.message)" }.joined(separator: "\n")
+        return HStack(alignment: .top, spacing: 8) {
+            Image(systemName: "exclamationmark.triangle.fill").foregroundStyle(.yellow)
+            Text(text).font(.caption).foregroundStyle(.white).lineLimit(errors.count + 1)
+            Spacer()
+            Button { Task { await onNavigate() } } label: {
+                Image(systemName: "arrow.clockwise").foregroundStyle(.white)
+            }
+        }
+        .padding(.horizontal, 12).padding(.vertical, 8)
+        .background(Color.red.opacity(0.85))
+    }
+
     // MARK: – Calendar content (with swipe)
 
     @ViewBuilder