Clean up Arch profile: remove i3/Void-specific config and fix GNOME integration

- Remove XFCE/i3-specific packages and dotfiles from Arch path (xarchiver,
  udiskie, Thunar, xfce-polkit, clipman, screenshooter)
- Separate per-OS dotfiles: mimeapps, udiskie config, GTK theme script,
  udiskie-password, dbus-session and ssh-agent fragments moved to Void-only
- Add mimeapps.arch.list with Nautilus/GNOME associations for nymph
- Move dunst/rofi directory creation from common to profile_desktop_i3
- Add gnome-keyring PAM hooks for GDM (gdm-password) in profile_desktop_gnome
- Remove ssh-agent.service from desktop_systemd_user_services on Arch;
  drop ssh-agent dependency and hardcoded socket from emacs.service
- Add ttf-hack-nerd to Arch font packages
- Fix rustup bootstrap: use rustup-init on Void, rustup toolchain install on Arch

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dotfiles/desktop/.config/systemd/user/emacs.service

@@ -1,11 +1,8 @@
 [Unit]
 Description=Emacs daemon
-After=ssh-agent.service
-Wants=ssh-agent.service
 
 [Service]
 Type=simple
-Environment=SSH_AUTH_SOCK=%h/.local/state/ssh-agent/socket
 WorkingDirectory=%h
 ExecStart=/usr/bin/emacs --fg-daemon
 Restart=on-failure