feat: gate XDG and PATH behind overrides toggle

Wrapped the XDG variables and PATH setup in config.fish with the C3 overrides toggle so users can fully opt out. Updated the documentation (07-customization.md, 02-path-setup.md, README.md) to reflect this. Also added a Home link to the 404 page and removed a leftover test_sidebar.js file.

README.md

@@ -279,7 +279,7 @@ If you'd rather set them by hand, each category is controlled by a universal var
 |---|---|
 | `__fish_config_op_aliases` | Command shadows: `ls`→eza, `cat`→bat, `cd`→zoxide, `rm`→trash, `top`→btop, `edit`→multi-editor launcher, and friends; `grep`/`cp`/`mv`/`wget` flag injection |
 | `__fish_config_op_autoexec` | Startup side-effects: Fisher bootstrap, theme apply, `paru`/`yay` wrapper generation, auto venv activation, WakaTime hook, auto-pull background fast-forward |
-| `__fish_config_op_overrides` | Vi mode, `exit`→`smart_exit`, `$PAGER`/`$MANPAGER`/`$CDPATH`, bang-bang history expansion, autopair, puffer, Starship prompt, theme colors |
+| `__fish_config_op_overrides` | Vi mode, `exit`→`smart_exit`, `$PAGER`/`$MANPAGER`/`$CDPATH`/`XDG`/`PATH`, bang-bang history expansion, autopair, puffer, Starship prompt, theme colors |
 | `__fish_config_op_integrations` | Kitty/WezTerm window abbreviations, `done` notifications, `spwin`/`tab`/`split`, `hist`, `logs`, `upgrade`, WakaTime |
 | `__fish_config_op_logging` | Scrollback capture on exit, tmux `pipe-pane` pane logging, zellij `dump-screen` capture on exit, `paru`/`yay` AUR log wrappers, Kitty watcher capture (sentinel-file coordinated) |
 | `__fish_config_op_greeting` | Per-session `fish_greeting` (suppresses distro greetings such as CachyOS fastfetch by overriding with an empty function); first-run welcome banner |