fix(calendar): use shared errorMessage() for PST import errors

Tauri commands reject with a {kind, message} object, not a native Error,
so `e instanceof Error ? e.message : String(e)` always fell through to
String(e) -> "[object Object]", hiding the actual readpst failure reason.

src/lib/stores/calendar.svelte.ts

@@ -1,7 +1,7 @@
 // Imported calendar events (Phase 6, FR-CAL-*). Svelte 5 runes store, same
 // shape as settings.svelte.ts/meetings.svelte.ts.
 
-import { api, events, type CalendarEvent } from "../api";
+import { api, errorMessage, events, type CalendarEvent } from "../api";
 
 class CalendarStore {
   events = $state<CalendarEvent[]>([]);
@@ -30,7 +30,7 @@ class CalendarStore {
       await api.importPst(path, password);
       await this.load();
     } catch (e) {
-      this.importError = e instanceof Error ? e.message : String(e);
+      this.importError = errorMessage(e);
     } finally {
       this.importing = false;
       this.importProgress = null;