feat(admin): signed customization page (custom.txt generator)

Adds an admin UI page that produces an Ed25519-signed custom.txt the
patched RustDesk client picks up at startup. The blob carries
app-name, default-settings, override-settings, an optional logo
image, and arbitrary buildin keys.

- gen_branding_sk + read_branding_sk + read_branding_pubkey in
  common.rs; called once on hbbs startup so the branding keypair
  (separate from the rendezvous id_ed25519) is created on first boot
  alongside it. Decoupling lets the two rotate independently.
- src/api/admin/pages/customization.rs: form with multipart logo
  upload (PNG/ICO/JPEG, magic-byte sniffed, capped at 256 KiB), JSON
  default/override settings fields, kv-pair rows for arbitrary
  buildin keys. Signs with sodiumoxide and emits standard-base64
  matching the client's decode64 path. Result block has a copyable
  blob and a "Download custom.txt" button.
- Routes wired under /admin/pages/customization{,/generate,/download}
  and the admin nav gets a Customization link.
- src/verify_branding.rs: smoke-test binary the operator can run
  before cutting a client release to confirm the signature
  round-trips.
- axum gains the multipart feature flag for the upload handler.

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

admin_ui/index.html

@@ -45,6 +45,8 @@
            hx-get="/admin/pages/recordings" hx-target="#main" hx-push-url="#recordings">Recordings</a>
         <a class="nav-link block px-3 py-1.5 text-sm rounded text-slate-300 hover:bg-slate-800"
            hx-get="/admin/pages/deploy" hx-target="#main" hx-push-url="#deploy">Deploy</a>
+        <a class="nav-link block px-3 py-1.5 text-sm rounded text-slate-300 hover:bg-slate-800"
+           hx-get="/admin/pages/customization" hx-target="#main" hx-push-url="#customization">Customization</a>
       </nav>
       <div class="px-2 py-3 border-t border-slate-800 space-y-1">
         <a class="nav-link block px-3 py-1.5 text-sm rounded text-slate-400 hover:bg-slate-800"