Implement Kitty graphics protocol for background image rendering

- Added `kitty_gfx` module to handle loading, scaling, and displaying background images using the Kitty graphics protocol.
- Integrated background image handling into the main application loop, allowing dynamic updates based on configuration.
- Enhanced terminal rendering to support transparent backgrounds when a Kitty image is active.
- Updated IPC server to manage client connections and messages, including handling background image settings.
- Modified `PtyTerminal` to accept additional environment variables during shell spawning.
- Improved rendering logic to support popup dialogs and terminal background color customization.

config.toml

@@ -1,5 +1,16 @@
 # TUI-WM konfiguration
 
+# Bakgrundsbild (Kitty graphics protocol — kräver Kitty/Ghostty/WezTerm)
+background_image = "/home/brasse/Bilder/backrunder/pexels-veeterzy-38136.jpg"
+
+# Bakgrundsfärg för virtuella terminaler.
+# Om inte satt → genomskinlig (bakgrundsbild syns igenom).
+# Stödjer: "#RRGGBB", "0"-"255" (indexed), färgnamn (black, red, etc.)
+# terminal_bg_color = "#1a1a2e"
+
+# Standard-shell för nya terminaler. Om inte satt → $SHELL eller /bin/bash.
+# default_shell = "/bin/fish"
+
 [[panel]]
 position = "top"
 file = "panels/topbar.toml"