feat: replace loupe with imv and update plymouth theme

- Replaced Loupe with imv for a keyboard-driven image viewer experience.

- Updated Plymouth logo with Firewatch-inspired ASCII art gradient.

- Updated Plymouth background color to match Firewatch palette (#2D112A).

modules/home/apps.nix

@@ -3,7 +3,7 @@
     ../../apps/yazi
     ../../apps/gemini
     ../../apps/firefox
-    ../../apps/loupe
+    ../../apps/imv
     ../../apps/mpv
     ../../apps/mako
     # ../../apps/starship