Add a power menu, so a session can be left without rebooting

The hyprcosmic profile disables cosmic-panel, and COSMIC's power applet lives
in that panel. Nothing replaced it, so the session had no logout, reboot or
shutdown anywhere in it: the only way out was `systemctl reboot` typed into a
terminal, which also meant every login-time change cost a reboot to test.

hyprcosmic-powermenu is a rofi menu offering lock, suspend, log out, reboot and
shut down. It is reached two ways -- $mainMod SHIFT E and a button at the right
end of waybar -- and both run the same script, so a click cannot bypass the
confirmation a keypress gets.

Logging out calls com.system76.CosmicSession.Exit, which is the method the
panel applet used and the only one that stops the session's clients in order
rather than pulling the compositor out from under them. Reboot and poweroff go
straight to systemd; polkit already authorises both for an active local session
without a prompt, verified with pkcheck, so no pkexec is involved. Lock uses
loginctl, which cosmic-greeter is listening for.

Confirmation is asked only for the three that end the session. Lock and suspend
undo themselves with a keypress, so a prompt there is pure friction; the other
three throw away everything unsaved and are one keystroke away at all times.
"No" is listed first so it is the row already selected.

The menu entries are plain words rather than Nerd Font glyphs. The bar icon is
the only glyph involved, and it comes through generate-config.py, whose whole
purpose is that no Private Use Area character is ever typed by hand -- U+F011,
confirmed present in the installed JetBrainsMono Nerd Font.

rules.css names every module id explicitly, so #custom-power had to be added
there too or the button would have rendered with no pill behind it.

The script sits under config/bin/ rather than a new top-level directory so that
install-assets.sh's audit still covers it: that check refuses to run unless
every file under config/ is classified, which is what stops a new file from
being silently left uninstalled.

tools/install-assets.sh

@@ -73,6 +73,7 @@ SHARED=(
     "config/waybar/rules.css:share/hyprcosmic/waybar/rules.css:644"
     "config/rofi/palette.rasi:share/hyprcosmic/rofi/palette.rasi:644"
     "config/rofi/rules.rasi:share/hyprcosmic/rofi/rules.rasi:644"
+    "config/bin/hyprcosmic-powermenu:bin/hyprcosmic-powermenu:755"
 )
 
 # The session entry point. Kept apart from SHARED because it is versioned in the