feat(menu): one fingerprint switch, an auto-login toggle, and the state bridge they needed

Bernardo, post-reboot: "Use for login" was the wrong question. Whether the
finger works is one decision, not two, and whether login prompts at all is
a different decision that was never in the menu.

System › Fingerprint is now a single Fingerprint (on/off) switch, leading
the menu with enroll/list/verify/delete as the plumbing behind it. It
writes the one settings.fingerprint.pam key, and modules/home/idle.nix now
defaults idle.fingerprint from that same key — so the lock screen and
login/sudo move together instead of drifting apart the way they did until
e2de906. nomarchy-fingerprint does the two rebuilds this needs (sudo
system for PAM, home switch for hyprlock) and refuses to turn on with no
finger enrolled.

System › Auto-login is new (nomarchy-autologin), and it is what decides
whether anything is asked at boot: auto-login on means no prompt whatever
the fingerprint switch says; off means the greeter asks, for a password or
a finger. Installer-seeded ON for LUKS machines — the passphrase already
gates the disk — and off without it, where the greeter is the only thing
between power-on and the desktop.

Both had to become state-owned to be toggleable at all, which surfaced two
real bugs:

  * nomarchy.system.greeter.autoLogin defaulted from
    `config.nomarchy.settings…` — an attribute that exists ONLY on the Home
    Manager side. On NixOS it is absent and `or null` swallowed the error,
    so the default silently evaluated to null on every machine ever built.
    That is why the installer baked a Nix line: the state path never
    worked. Now read via theme-state-read.nix (the hardware.nix/timezone.nix
    pattern) and mkDefault'd, so the menu owns it and a hand-set line still
    pins it. Two more options read the same phantom bridge — BACKLOG #116.
  * `theme-sync get` printed Python's "None" for a JSON null, so every
    `case … null)` a caller writes would miss. Now prints "null", as the
    comment above it already promised for booleans.

The installer seeds the state instead of emitting the system.nix line,
because that line outranks the state and would strand the toggle.

V1 (V3 pending: HARDWARE-QUEUE). nix flake check --no-build, installer-
safety and option-docs all pass. Proved by eval/build, not assumed: a state
carrying autoLogin yields greetd initial_session {"user":"bernardo"}, the
template state (no autoLogin) yields none, and a hand-set null beats a state
that says otherwise; a state with only fingerprint.pam=true — nothing set by
hand — renders the hyprlock auth.fingerprint block; both new tools pass
bash -n and land in systemPackages (nomarchy-fingerprint only with a
reader); the patcher writes settings.greeter.autoLogin and no system.nix
line; and the get round trip prints null, so the menu reads "Auto-login
(off)" where it would have read "(on)".

The reader itself, the two rebuilds, and the reboot are hardware — queued.

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

templates/downstream/home.nix

@@ -18,12 +18,13 @@
   # nomarchy.waybar.enable = false;     # bring your own bar
   # nomarchy.stylix.enable = false;     # opt out of GTK/Qt theming
   # nomarchy.dockAudio.enable = false;  # keep audio put when docking
-  # nomarchy.idle.fingerprint = true;   # unlock the lock screen by finger too, and
-  #                                     # say so on the field. Set it alongside
-  #                                     # nomarchy.hardware.fingerprint.pam in
-  #                                     # system.nix — hyprlock is configured here
-  #                                     # and cannot read the NixOS side, and it
-  #                                     # does not take a finger through PAM at all.
+  # nomarchy.idle.fingerprint = true;   # pin lock-screen finger unlock on/off. Normally
+  #                                     # you want System › Fingerprint instead: it moves
+  #                                     # this AND login/sudo together off one state key,
+  #                                     # and this line would override it. Separate options
+  #                                     # only because hyprlock is configured here (it can't
+  #                                     # read the NixOS side) and doesn't take a finger
+  #                                     # through PAM at all — it has its own fprintd backend.
   #
   # Icon pack: Papirus ships by default (auto Dark/Light by theme mode) and
   # is the ONLY icon pack in your closure. To use another, set the `icons`

templates/downstream/system.nix

@@ -62,7 +62,9 @@
   #   amd.rocm.enable = true;                   #   ROCm GPU compute (multi-GB, opt-in)
   #   amd.rocm.gfxOverride = "11.0.0";          #   HSA override for an unlisted iGPU
   #   fingerprint.enable = true;                #   fprintd; installer-set when a reader is detected
-  #   fingerprint.pam = true;                   #   use the fingerprint for login + sudo
+  #   fingerprint.pam = true;                   #   pin the fingerprint on/off instead of letting the
+  #                                             #   System › Fingerprint toggle own it (this line WINS
+  #                                             #   over the menu — leave it commented to use the menu)
   #   fingerprint.parallel = false;             #   sequential prompt instead of password-or-finger
   #   npu.enable = true;                        #   on-die NPU driver (experimental; userspace runtime BYO)
   #   latestKernel = true;                       #   newest kernel for very-new hardware (drivers not yet in the default)