launcher-menu: grid of tiles and hicolor icon fallbacks

launcherMenu.grid = { enable; columns; maxRows; } restyles the "+" menu's
programs section as a grid of tiles (icon, name below): Steam's own items
stay in place, maxRows limits the visible rows, and Steam's scroll fade is
recomputed so it only shows where there is more to scroll. The pinned
Desktop row is slim and centred in tile mode.

launcherMenu.iconFallbacks (default utilities-terminal, preferences-system)
installs hicolor copies of Breeze app icons, since Steam resolves program
icons only in hicolor; Konsole and KDE System Settings had none.

The checker gains checkOnly "stylesheet" signatures (matched against the
bundle's CSS files) to cover the scroll fade's gradient; launcher-menu-grid
signatures cover the section keys and the ScrollFade classes.

template/home.nix

@@ -18,12 +18,14 @@
   #   keyboardLayout = "de";             # XKB layout for the Steam session
   #   steamKeyboardPatch.enable = true;  # Esc/Ctrl/Alt/arrows on the VR keyboard
   #   # VR "+" menu: sorted by name, Desktop pinned below the list, closed
-  #   # on click, no second launch of the same program within 10 s:
+  #   # on click, no second launch of the same program within 10 s, programs
+  #   # as a grid of 4 columns, at most 4 rows visible:
   #   launcherMenu = {
   #     sort = true;
   #     pinDesktop = "bottom";
   #     closeOnLaunch = true;
   #     launchDebounce = 10;
+  #     grid = { enable = true; columns = 4; maxRows = 4; };
   #   };
   #   # SteamVR dashboard windows: resizable up to 4x (stock 2x), pushed
   #   # back up to 10 m in the world / 12 m in theater mode (stock 5 / 6 m):