add webview map, bike reservation, and desktop entry

- Replace list view with Leaflet.js WebView map with marker clustering
- Add floating action buttons over map (Rent, Show Rentals)
- Click station markers to show bikes with Rent/Reserve buttons
- Add bike type filter menu (All/Standard/E-bikes)
- Support bike reservations via booking API
- Show reserved bikes in rentals overview
- Add desktop entry for app launchers
- Update dependencies: webkit6 0.6, gtk4 0.11, libadwaita 0.9

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -16,6 +16,7 @@
         gtk4
         libadwaita
         glib
+        webkitgtk_6_0
       ];
       buildDeps = with pkgs; [
         pkg-config
@@ -40,6 +41,8 @@
         postInstall = ''
           install -Dm644 data/icons/org.nextbike.NextCompanion.png \
             $out/share/icons/hicolor/512x512/apps/org.nextbike.NextCompanion.png
+          install -Dm644 data/org.nextbike.NextCompanion.desktop \
+            $out/share/applications/org.nextbike.NextCompanion.desktop
         '';
       };