Add custom theme support with wallpaper-based color extraction (#6)

* Add custom theme support with wallpaper-based color extraction

- Extended theme option to accept "custom" value
- Added customTheme configuration with wallpaperPath and variant options
- Integrated nix-colors colorSchemeFromPicture for dynamic color generation
- Updated wallpaper selection to handle custom paths
- Added VSCode theme fallback for custom color schemes
- Fixed nix-colors contrib function call (requires pkgs argument)
- Maintains full backward compatibility with existing themes

Users can now generate themes from their own wallpapers:
  omarchy = {
    theme = "custom";
    customTheme = {
      wallpaperPath = ./wallpapers/my-image.png;
      variant = "dark"; # or "light"
    };
  };

* Fix: Screen sharing broken due to Qt library version mismatch

  File changed: modules/nixos/hyprland.nix

  Problem:
  The omarchy-nix configuration was using xdg-desktop-portal-hyprland from the hyprland flake input, which had a Qt library version mismatch (6.9.1 vs 6.9.0). This caused the
   hyprland-share-picker to crash with "Cannot mix incompatible Qt library" errors, preventing screen sharing from working.

  Solution:
  Changed the portalPackage from the hyprland flake version to the stable nixpkgs version:

  - portalPackage = inputs.hyprland.packages.${pkgs.stdenv.hostPlatform.system}.xdg-desktop-portal-hyprland;
  + portalPackage = pkgs.xdg-desktop-portal-hyprland; # Use stable nixpkgs version to fix Qt version mismatch

  Why this works:
  - The stable nixpkgs version of xdg-desktop-portal-hyprland has consistent Qt library versions
  - This matches the working configuration used on other NixOS Hyprland setups
  - Maintains all screen sharing functionality while avoiding the Qt version conflict

modules/home-manager/default.nix

@@ -7,7 +7,19 @@ inputs: {
   packages = import ../packages.nix {inherit pkgs lib; exclude_packages = config.omarchy.exclude_packages;};
 
   themes = import ../themes.nix;
-  selectedTheme = themes.${config.omarchy.theme};
+  
+  # Handle theme selection - either predefined or custom
+  selectedTheme = if config.omarchy.theme == "custom"
+    then themes.custom
+    else themes.${config.omarchy.theme};
+  
+  # Generate color scheme - either from predefined or from wallpaper
+  generatedColorScheme = if config.omarchy.theme == "custom"
+    then (inputs.nix-colors.lib.contrib { inherit pkgs; }).colorSchemeFromPicture {
+      path = config.omarchy.customTheme.wallpaperPath;
+      variant = config.omarchy.customTheme.variant;
+    }
+    else null;
 in {
   imports = [
     (import ./hyprland.nix inputs)
@@ -35,7 +47,9 @@ in {
   };
   home.packages = packages.homePackages;
 
-  colorScheme = inputs.nix-colors.colorSchemes.${selectedTheme.base16-theme};
+  colorScheme = if config.omarchy.theme == "custom"
+    then generatedColorScheme
+    else inputs.nix-colors.colorSchemes.${selectedTheme.base16-theme};
 
   gtk = {
     enable = true;