DRY pass #2, batch 7, F21: the desktop's main window, clock and launcher dir (#5372)

- lib.rs MAIN_WINDOW: the "main" window label that the reminder worker,
  the capture window and the shell each wrote, 5 sites in all.
- lib.rs now_ms(): the epoch-milliseconds clock that the reminder worker
  and autosync's cycle stamp each computed. LastCycle.at_ms becomes i64,
  the same number on the wire.
- integration.rs applications_dir(): the XDG launcher directory that the
  entry path and the desktop-database refresh each built.

rustfmt --check is clean in the CI image.

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

desktop/src-tauri/src/autosync.rs

@@ -46,7 +46,7 @@
 
 use std::sync::mpsc::{self, Receiver, RecvTimeoutError, Sender};
 use std::sync::Mutex;
-use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
+use std::time::{Duration, Instant};
 
 use serde::Serialize;
 use tauri::{AppHandle, Emitter, Manager};
@@ -85,7 +85,7 @@ pub enum Trigger {
 #[derive(Debug, Clone, Serialize)]
 pub struct LastCycle {
     /// When it finished, in milliseconds since the epoch — the frontend formats it.
-    pub at_ms: u64,
+    pub at_ms: i64,
     pub trigger: Trigger,
     pub ok: bool,
     /// Why it failed, in words a person can act on. None when `ok`.
@@ -299,9 +299,7 @@ fn worker(app: AppHandle, rx: Receiver<Request>) {
         }
 
         let cycle = LastCycle {
-            at_ms: SystemTime::now()
-                .duration_since(UNIX_EPOCH)
-                .map_or(0, |d| d.as_millis() as u64),
+            at_ms: crate::now_ms(),
             trigger,
             ok: result.is_ok(),
             error: result.as_ref().err().cloned(),