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.

docs/why-ferese.md

@@ -10,7 +10,7 @@ Scrolling columns let you keep windows at useful widths and move through them
 horizontally. Tree tiling divides the screen into nested splits. Floating windows
 work alongside either layout, and overview shows windows across workspaces.
 
-![Ferese Overview showing windows and workspaces](images/ferese-overview.webp)
+![Ferese Overview showing windows and workspaces](images/screenshots/ferese-blue-overview.png)
 
 ## Change the desktop together
 
@@ -18,7 +18,7 @@ Choose a theme in Settings, then adjust the wallpaper, colors, font, corners,
 and motion. Changes apply live. Window corners and shell corners can be set
 independently.
 
-![Ferese Settings and Control Center in Gruvbox](images/ferese-gruvbox.webp)
+![Ferese Settings and Control Center in Rosé Pine](images/screenshots/rose-pine-desktop.png)
 
 For example, this changes shell corners: