Reorganize desktop roles: extract common bootstrap and host-specific layers

- Add profile_desktop_common with shared desktop bootstrap (emptty, PAM,
  dotfiles, templates, GPG, Maildir, Flatpak, st, external tools)
- Add profile_desktop_host with host-specific tasks (NVIDIA/PRIME on nymph)
- Reduce profile_desktop_i3 to i3/X11-only tasks
- Create profile_desktop_hyprland for Hyprland Wayland session
- Add dual-session support (i3 + Hyprland) on nymph with session choice
- Create shared Hyprland/Waybar dotfiles under dotfiles/desktop/
- Fix Waybar: bottom position, no persistent workspaces, sort by number
- Rename host_dotfiles to host_i3_dotfiles for clarity
- Make emptty restart manual by default to avoid session drops

dotfiles/desktop/.local/bin/start-hyprland-session

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+[ $# -gt 0 ] || set -- Hyprland
+
+[ -r /etc/profile ] && . /etc/profile
+[ -r "$HOME/.profile" ] && . "$HOME/.profile"
+
+set -e
+
+session_name=${1##*/}
+
+export XDG_CURRENT_DESKTOP="$session_name"
+export XDG_SESSION_DESKTOP="$session_name"
+export XDG_SESSION_TYPE=wayland
+
+exec dbus-run-session sh -eu -c '
+  umask 077
+  printf "%s\n" "$DBUS_SESSION_BUS_ADDRESS" > "$HOME/.dbus-session-bus-address"
+  eval "$(ssh-agent -s)" >/dev/null
+  gpgconf --launch gpg-agent
+  exec "$@"
+' sh "$@"