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>

core/src/local/models.rs

@@ -95,6 +95,19 @@ pub struct TitleEntry {
     pub title: String,
 }
 
+/// A reminder that has come due, as the desktop's reminder worker needs it.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
+pub struct DueReminder {
+    pub id: String,
+    pub title: String,
+    /// `remind_at` as stored. It names the occurrence: completing or snoozing a
+    /// reminder changes it, and that is how an announcer tells a new occurrence
+    /// from one it has already announced.
+    pub remind_at: String,
+    /// The same instant, in milliseconds since the epoch.
+    pub due_ms: i64,
+}
+
 #[derive(Serialize)]
 pub struct SavedFilter {
     pub id: String,