Fix wallpaper persistence, tray icons, Booru bugs, session launcher

- Persist last-set wallpaper and restore it via exec-once, since
  hyprpaper/mpvpaper start every session with nothing loaded
- Fix Vesktop's system tray icon rendering as a broken texture: only
  resolve tray icons through the icon theme when they're actual names,
  not raw pixmap-sourced image:// URLs
- Launch Hyprland via start-hyprland (its official watchdog launcher)
  instead of the raw binary, removing the startup warning and adding
  hang recovery
- Fix Booru panel: Lewds toggle now actually persists, tag autocomplete
  no longer drops/races overlapping requests, and pagination wraps
  instead of overflowing off-screen
- Remove the redundant gap between windows and sidebars/tiles
  (gaps_in/gaps_out, and duplicate flyout margins stacked on top of
  Hyprland's automatic exclusive-zone inset)

modules/desktop.nix

@@ -11,7 +11,13 @@
   services.greetd = {
     enable = true;
     settings.default_session = {
-      command = "${pkgs.tuigreet}/bin/tuigreet --time --remember --remember-session --cmd Hyprland";
+      # start-hyprland, not the raw Hyprland binary: it's Hyprland's own
+      # watchdog-wrapped launcher (ships alongside Hyprland in the same
+      # package) that also auto-restarts the compositor if it hangs.
+      # Launching the raw binary directly works but logs a startup warning
+      # ("Hyprland is being launched without start-hyprland. This is highly
+      # advised against.") and skips that recovery.
+      command = "${pkgs.tuigreet}/bin/tuigreet --time --remember --remember-session --cmd start-hyprland";
       user = "greeter";
     };
   };