feat(machine): operator branding — local-file source (issue #47 V1)

Read /var/lib/bitspire/branding/{logo.png,branding.json} on startup and
apply across the renderer. branding.json may set title, theme (one of
the 6 built-ins or "custom"), and a custom_colors map (with optional
.dark overlay) — unset CSS vars fall back to gruvbox.

Wiring:
- electron/main.ts:loadBranding() reads + validates the JSON and
  base64-encodes logo.png; surfaced via the existing get-config IPC
- composables/useBranding.ts holds reactive logoUrl/title refs and a
  single setBranding() setter — the seam where #48's Nostr-event
  source will eventually overlay the local-file source
- composables/useTheme.ts:applyBrandingTheme() handles built-in theme
  swap and injects a <style#branding-custom-theme> block for custom
- IdleView binds :src/title; App.vue calls setBranding() before the
  maintenance screen renders so "Under Service" wears operator branding

Provisioning: new deploy/nixos/provision-branding.sh rsyncs a local dir
to /var/lib/bitspire/branding/ via sudo-on-the-far-side and restarts
bitspire.service. The existing provision-atm.sh stays focused on .env.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/nixos/README.md

@@ -157,6 +157,7 @@ Production ATMs on `main` continue to read `main`'s flake (no `?ref=` pin → re
 | `/var/lib/bitspire/.env` | bitspire:bitspire, 0600 | Runtime config — `VITE_RELAY_URL`, `VITE_LNBITS_SERVER_PUBKEY`, `VITE_LNBITS_HTTP_URL`, `VITE_ATM_PRIVATE_KEY`, … |
 | `/var/lib/bitspire/state.db` | bitspire:bitspire | SQLite — cassette inventory, cashbox state, transaction history |
 | `/var/lib/bitspire/logs/` | bitspire:bitspire, 0750 | Service logs (if app writes them) |
+| `/var/lib/bitspire/branding/` | bitspire:bitspire, 0755 | Operator branding override (logo.png + branding.json) — see issue #47 |
 | `/opt/bitspire/` | bitspire:bitspire | Optional override drop for app assets (mostly unused — app comes from `/nix/store`) |
 | `/etc/bitspire/config.env` | root:root | Static config emitted by the NixOS module (RELAY_URL, LNBITS_HTTP_URL — informational; the renderer reads `/var/lib/bitspire/.env` instead) |