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/Services/CalendarrAPI.swift

@@ -206,7 +206,12 @@ class CalendarrAPI {
 
     // MARK: – Events
 
-    func fetchEvents(start: Date, end: Date) async throws -> [CalEvent] {
+    /// Fetches events for the personal calendar view. The server also reports
+    /// per-calendar sync failures (e.g. expired credentials) alongside an
+    /// otherwise-successful response, via the `errors` array — surfaced
+    /// separately from a hard fetch failure so the UI can distinguish
+    /// "everything failed" from "fetched fine, but calendar X didn't sync".
+    func fetchEvents(start: Date, end: Date) async throws -> (events: [CalEvent], errors: [SyncError]) {
         // Use UTC with Z suffix – avoids '+' character which breaks URL query params
         let iso = ISO8601DateFormatter()
         iso.formatOptions = [.withInternetDateTime]
@@ -224,7 +229,9 @@ class CalendarrAPI {
             let preview = String(data: data, encoding: .utf8).map { String($0.prefix(200)) } ?? "no data"
             throw APIError.serverError("Unerwartete Antwort: \(preview)")
         }
-        return arr.compactMap { CalEvent.from(json: $0) }
+        let events = arr.compactMap { CalEvent.from(json: $0) }
+        let errors = (root["errors"] as? [[String: Any]])?.compactMap { SyncError.from(json: $0) } ?? []
+        return (events, errors)
     }
 
     func createLocalEvent(calendarId: Int, title: String, start: Date, end: Date,