gui: set app_id so Wayland compositors match the desktop icon

WindowOptions.app_id was left unset, so KWin (and likely other Wayland
compositors) had nothing to match against synchronicity.desktop's
Icon= for surfaces like Alt-Tab — they fell back to a generic icon
even though the taskbar (which matches more loosely) showed the right
one. Set app_id to "synchronicity", matching the desktop file's base
name, and add StartupWMClass for the same match on X11/XWayland.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/synchronicity.desktop

@@ -3,5 +3,6 @@ Name=Synchronicity
 Comment=Personal Data Store
 Exec=syn-gui
 Icon=synchronicity
+StartupWMClass=synchronicity
 Type=Application
 Categories=Utility;Network;

rust/gui-app/src/main.rs

@@ -260,6 +260,13 @@ fn main() {
                     ..Default::default()
                 }),
                 window_bounds: Some(WindowBounds::Windowed(bounds)),
+                // Must match the `synchronicity.desktop` file's base name so
+                // Wayland compositors (e.g. KWin) can look up its `Icon=` for
+                // window-switcher/Alt-Tab surfaces — without this the window
+                // has no app id to match against and falls back to a generic
+                // icon there, even though the taskbar (which matches more
+                // loosely) shows the right one.
+                app_id: Some("synchronicity".to_string()),
                 ..Default::default()
             };
             if let Err(err) = cx.open_window(window_options, |window, cx| {