refactor: implement component-based architecture for enhanced maintainability

- Reorganize directory structure into core/, features/, and themes/
- Colocate application Nix logic, configs, scripts, and theme overrides
- Implement 'Inversion of Control' for theming: apps now pull theme-specific layouts
- Update flake.nix and shared library paths to match the new structure
- Document the new Feature-Centric architecture in README.md

themes/templates/ghostty.conf.tpl

@@ -0,0 +1,22 @@
+background = {{ background }}
+foreground = {{ foreground }}
+cursor-color = {{ cursor }}
+selection-background = {{ selection_background }}
+selection-foreground = {{ selection_foreground }}
+
+palette = 0={{ color0 }}
+palette = 1={{ color1 }}
+palette = 2={{ color2 }}
+palette = 3={{ color3 }}
+palette = 4={{ color4 }}
+palette = 5={{ color5 }}
+palette = 6={{ color6 }}
+palette = 7={{ color7 }}
+palette = 8={{ color8 }}
+palette = 9={{ color9 }}
+palette = 10={{ color10 }}
+palette = 11={{ color11 }}
+palette = 12={{ color12 }}
+palette = 13={{ color13 }}
+palette = 14={{ color14 }}
+palette = 15={{ color15 }}