fix(theme): set default to summer-night and fix scripts on live ISO

- Update lib/state-schema.nix to default both home and system themes to 'summer-night'.
- Fix 'nomarchy-theme-list' and 'nomarchy-theme-set-templates' to resolve themes and templates from '~/.local/share/nomarchy' instead of the obsolete '$NOMARCHY_PATH' (fixing failures on Live ISO).
- Update 'nomarchy-welcome' to properly convert Title Case theme display names back to kebab-case identifiers and add input validation to prevent crashes.
- Fix installer impermanence symlink by using a relative path ('../persist/etc/nixos'), ensuring it resolves during 'nixos-install' both inside and outside the chroot.
- Deploy '~/.XCompose' symlink via Home Manager and add 'nomarchy-restart-xcompose' to the menu.
- Relocate 'Nomarchy.ttf' to 'core/branding/' and move user-level scripts ('pkg-add', 'pkg-remove', 'env-update', 'preflight-migration') to 'features/scripts/utils/' to align with the distro architecture.
- Remove obsolete '$NOMARCHY_PATH' exports and redundant 'bashrc' template.
- Export theme templates via 'xdg.dataFile' for script accessibility.

lib/state-schema.nix

@@ -6,7 +6,7 @@
   # Home state defaults (user preferences)
   home = {
     # Theme and appearance
-    theme = "nord";
+    theme = "summer-night";
     wallpaper = "";
     font = "JetBrainsMono Nerd Font";
     panelPosition = "top";
@@ -31,7 +31,7 @@
   # System state defaults (system-level configuration)
   system = {
     # Theme (can differ from home for system-level theming)
-    theme = "nord";
+    theme = "summer-night";
 
     # Timezone
     timezone = "UTC";