feat(power): laptop power management (power-profiles-daemon + TLP)

Adds a `nomarchy.system.power.*` surface (modules/nixos/power.nix),
replacing the bare `services.upower` (Waybar reporting only) baseline:

- power-profiles-daemon ships by default; switched through polkit (no
  root prompt). TLP is the opt-in `backend = "tlp"` for deeper battery
  tuning — the two are mutually exclusive (asserted).
- thermald behind `power.thermal.enable` (Intel-only).
- `power.batteryChargeLimit` (e.g. 80): a backend-independent sysfs
  oneshot writes charge_control_end_threshold; gated on `power.laptop`.

Switcher + indicator live home-side and self-gate on a battery being
present + powerprofilesctl running, so they hide on desktops and under
TLP — no system->home wiring (like the Waybar battery widget):

- rofi.nix: `nomarchy-menu power-profile` module + a picker entry shown
  only on laptops with PPD.
- waybar.nix: first `custom/*` module — click cycles the profile.

Installer writes `power.laptop = true` (battery probe) and
`power.thermal.enable` (GenuineIntel) into the generated system.nix, and
scaffolds the charge limit commented-out.

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

modules/nixos/default.nix

@@ -22,7 +22,7 @@ let
   '';
 in
 {
-  imports = [ ./options.nix ./plymouth.nix ./file-manager.nix ];
+  imports = [ ./options.nix ./plymouth.nix ./file-manager.nix ./power.nix ];
 
   config = {
     # The safe half of distro branding: distroName flows into