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/installation.md

@@ -306,6 +306,9 @@ greeter's configuration. Reinstalling restores the desktop session entry.
 
 ## Screen sharing and recording
 
-The installer includes Ferese’s native ScreenCast portal. See
+The installer includes Ferese’s native ScreenCast portal. It backs up the old
+user service override that points to `~/.local/libexec/ferese`, so new sessions
+use the installed release. Custom overrides are preserved and reported before
+building. See
 [Screen sharing](screen-sharing.md) for consent, PipeWire setup, supported sources,
 and current recording limitations.