desktop: reminders reach you when the window isn't in front

A Rust worker reads due reminders from the local store every 15 s and
announces each occurrence once: always to the main window as a toast, and
as a system notification (tauri-plugin-notification) when that window
isn't focused. The page no longer polls on the desktop; its Notification
went nowhere in WebKitGTK and its timer stopped with the window. The
Reminders page says what each surface actually does.

Core gains store::due_reminders, compared by instant, not by string.

Refs #5171

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/views/RemindersView.vue

@@ -7,9 +7,16 @@ import AsyncState from "../components/AsyncState.vue";
 import EmptyState from "../components/EmptyState.vue";
 import NoteEditor from "../components/NoteEditor.vue";
 import NoteGrid from "../components/NoteGrid.vue";
+import { isDesktop } from "../desktop/bridge";
 
 const reminders = useReminderStore();
 
+// What this surface actually does with a due reminder. The desktop's worker raises
+// system notifications while it runs; a browser tab can only alert while it is open.
+const deliveryNote = isDesktop()
+  ? "Due reminders show as system notifications while Inkwell is running, even minimised."
+  : "Due reminders pop up while this tab is open. The desktop and Android apps alert you in the background.";
+
 const { items, loading, error, load } = useNoteList(() => reminders.fetchReminders(), "Couldn't load reminders.");
 
 const { editing, open: openEditor, close: closeEditor, navigate: onNavigate } = useNoteEditor({
@@ -37,9 +44,7 @@ onMounted(load);
       </button>
       <span v-else-if="reminders.osEnabled" class="text-xs text-neutral-400">Notifications on ✓</span>
     </div>
-    <p class="mb-5 text-xs text-neutral-400">
-      Due reminders pop up while Inkwell is open. Background alerts arrive with the desktop &amp; mobile apps.
-    </p>
+    <p class="mb-5 text-xs text-neutral-400">{{ deliveryNote }}</p>
 
     <AsyncState :loading="loading" :error="error || undefined" error-title="Couldn't load reminders" @retry="load">
       <EmptyState