refactor(config): rename VITE_LAMASSU_* machine-config env vars to VITE_BITSPIRE_*

MACHINE_MODEL, FIAT_CODE, VALIDATOR_DEVICE, DISPENSER_DEVICE and CASSETTES
carried the old brand in their names. Renamed everywhere they are read
(device.ts, electron/main.ts), written (flake.nix, mkAtmApp.nix, live.nix,
provision-atm.sh, factory-reset-atm.sh) and documented (.env.example,
docs/device-configuration.md). No compatibility fallback in code: the
machine reads VITE_BITSPIRE_* and nothing else.

The deployed .env files are the one place the old names persist — sintra's
/var/lib/bitspire/.env holds all three keys today — and the machine reads
MACHINE_MODEL / FIAT_CODE / CASSETTES from that file on every boot. Renaming
the keys in code alone would boot a live machine on preset defaults (wrong
bays, wrong fiat) at the next nightly pull. So configuration.nix gains an
activation script, beside the existing lamassu→bitspire user migration,
that rewrites VITE_LAMASSU_* → VITE_BITSPIRE_* in that file. Idempotent;
runs before bitspire.service starts.

flake.nix

@@ -335,8 +335,8 @@
                 mkdir -p /var/lib/bitspire
                 if [ ! -f /var/lib/bitspire/.env ]; then
                   cp ${pkgs.writeText "bitspire-env-default" (''
-                    VITE_LAMASSU_MACHINE_MODEL=${machineModel}
-                    VITE_LAMASSU_FIAT_CODE=${fiatCode}
+                    VITE_BITSPIRE_MACHINE_MODEL=${machineModel}
+                    VITE_BITSPIRE_FIAT_CODE=${fiatCode}
                     VITE_SPIRE_SEED=
                     ELECTRON_FORCE_PROD=1
                     DISPLAY=:0