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/capture.rs

@@ -214,7 +214,7 @@ pub fn capture_done(saved: bool, app: AppHandle) -> Result<(), String> {
         window.hide().map_err(|e| e.to_string())?;
     }
     if saved {
-        if let Some(main) = app.get_webview_window("main") {
+        if let Some(main) = app.get_webview_window(crate::MAIN_WINDOW) {
             // Failure here is cosmetic — the note is saved either way and the board
             // will show it on its next load — so it is logged, not raised.
             if let Err(e) = main.emit(CAPTURED_EVENT, ()) {