feat(idle): a real lock bind, and a lock screen that speaks in sentences

Two nits from the post-reboot hardware pass.

Super+Escape never locked anything — there was no lock bind at all, only
the power menu's Lock row and the idle timeout. (Escape is unusable here
anyway; keybinds.nix has said so since the power menu moved to Super+X,
and an unswallowed Escape just prints its sequence into the terminal.)
Lock now lives at Super+Ctrl+L: the one row in that family that is not a
menu module, because bare Super+L collides with common app binds. It
dispatches loginctl lock-session, the same path the power menu's Lock row
takes, so logind marks the session locked and hypridle's lock_cmd remains
the single place that decides what a lock looks like.

The lock prompt read as clipped fragments ("password… or scan finger" /
"scanning finger…") next to a 64px clock. Both messages are sentences
now, and the input field's placeholder is sentence-cased to match — the
two can share the screen, so they cannot read as two different voices.

V1 + V3 pending: nix flake check --no-build passes; the built home
generation carries `bind=$mod CTRL, L, exec, loginctl lock-session`, the
new ready/present messages (built with idle.fingerprint forced on, since
the in-repo config leaves it off), and a "SUPER + CTRL + L  Lock screen"
row the cheatsheet generated on its own from the shared list. The reader
itself only exists on hardware.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/OMARCHY.md

@@ -67,6 +67,7 @@ live cheatsheet — you never have to memorize this table.
 | `SUPER+SHIFT+T` | Next wallpaper |
 | `SUPER+E` | File manager (yazi) |
 | `SUPER+X` | Power menu (lock / suspend / hibernate / reboot …) |
+| `SUPER+CTRL+L` | Lock screen |
 | `SUPER+?` | Keybindings cheatsheet |
 | `Print` | Screenshot region → clipboard |
 | `SUPER+CTRL+<key>` | Jump straight to a menu module (clipboard `V`, calc `C`, emoji `E`, capture `S`, …) |