Refresh screenshot galleries and repair portal upgrades

Use the September 30 hero on the website and README, add a shared screenshot gallery, and remove the old showcase images. Build image links locally and clean generated site output.

Back up the known development portal override during installation and report custom overrides before building. Keep source selections clear while letting blur show through unselected rows.

site/content/index.md

@@ -3,6 +3,8 @@
 Ferese is a Linux Wayland desktop with scrolling columns, tree tiling, floating
 windows, and a shared theme for its desktop tools.
 
+[Browse screenshots](screenshots.md) of the desktop, themes and shell controls.
+
 ## Get started
 
 1. [Install Ferese](installation.md), or try a [nested preview](installation.md#preview-and-logs).