feat: replace Debian Live with custom debootstrap ISO base + branding

Major ISO build overhaul on dev-iso branch:

- Replace ~800MB Debian Live download with debootstrap --variant=minbase
  (~150MB installer squashfs built from scratch)
- Custom initramfs with archipelago-mount hook for boot media detection
- Systemd service auto-starts installer (replaces profile.d hack)
- GRUB + ISOLINUX configs written from scratch (no Debian Live dependency)
- EFI boot image built with grub-mkimage (no more MBR extraction)
- Archipelago GRUB theme: dark background, Bitcoin orange accents
- Theme installed on both installer ISO and target system
- Rootfs optimizations: --no-install-recommends, strip docs/man/locales,
  remove firmware-misc-nonfree/wget/htop, add explicit font deps
- Separate CI workflow (build-iso-dev.yml) for dev-iso branch
- Includes pre-existing fixes from main (build-iso.yml, middleware, Login)

Target: sub-2GB unbundled ISO (down from 3.9GB)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/middleware.rs

@@ -11,11 +11,14 @@ pub(super) const UNAUTHENTICATED_METHODS: &[&str] = &[
     "auth.setup",
     "auth.onboardingComplete",
     "health",
+    // Server readiness check (Login.vue polls this before showing form)
+    "server.echo",
     // Onboarding flow (before user has a session — DID creation, signing, backup)
     "node.did",
     "node.signChallenge",
     "node.nostr-pubkey",
     "node.createBackup",
+    "identity.create",
     "identity.verify",
     "identity.resolve-did",
     // Onboarding restore (before user account exists)