Use real KAMOS branding: red wallpaper, penguin logo + Plymouth boot watermark

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -1,2 +1,3 @@
 output/
 *.iso
+branding/

build_files/build.sh

@@ -35,13 +35,24 @@ if [ -f "$WALL" ]; then
 fi
 
 ### ---------------------------------------------------------------
-### 4. First-boot Flatpak apps (list: /usr/share/kamos/flatpaks.list)
+### 4. Boot splash: KAMOS penguin as Plymouth watermark
+### ---------------------------------------------------------------
+MARK=/usr/share/kamos/plymouth-watermark.png
+if [ -f "$MARK" ]; then
+    for theme in spinner bgrt; do
+        tdir=/usr/share/plymouth/themes/$theme
+        [ -d "$tdir" ] && cp -f "$MARK" "$tdir/watermark.png"
+    done
+fi
+
+### ---------------------------------------------------------------
+### 5. First-boot Flatpak apps (list: /usr/share/kamos/flatpaks.list)
 ### ---------------------------------------------------------------
 chmod +x /usr/libexec/kamos-install-flatpaks
 systemctl enable kamos-flatpaks.service
 
 ### ---------------------------------------------------------------
-### 5. System defaults
+### 6. System defaults
 ### ---------------------------------------------------------------
 # Enable weekly automatic OS updates (staged, applied on reboot)
 systemctl enable rpm-ostreed-automatic.timer || true