release(v1.7.36-alpha): bitcoin-core in App Store + Sovereignty Stack + dynamic catalog URL

- neode-ui/public/assets/img/app-icons/bitcoin-core.svg (NEW): 256×256
  Umbrel community Bitcoin icon sourced from getumbrel.github.io/
  umbrel-apps-gallery/bitcoin/icon.svg. Referenced by the static
  catalog, the curated fallback, and the upstream lfg2025/app-catalog
  entry so every surface shows the same image.
- app-catalog/catalog.json + neode-ui/public/catalog.json: add
  bitcoin-core (v28.4) entry pointing at bitcoin/bitcoin:28.4. Same
  entry pushed to the lfg2025/app-catalog repo on .160 and the local
  gitea mirror so nodes see it without needing a full archipelago
  update. Sovereignty Stack entry added to FEATURED_DEFINITIONS with
  a description that frames it as a Knots alternative, not a rival.
- core/archipelago/src/api/handler/mod.rs: handle_app_catalog_proxy
  is now instance-scoped (&self) and derives its upstream list from
  load_registries — each active container registry contributes one
  `<scheme>://<reg.url>/app-catalog/raw/branch/main/catalog.json` URL
  in priority order (scheme follows tls_verify). When the operator
  switches mirrors in Settings, the App Store now follows. Falls back
  to the legacy hardcoded .160/tx1138 pair only when registry config
  can't be loaded, so the App Store still renders on nodes that
  haven't persisted one yet.

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

neode-ui/public/catalog.json

@@ -22,6 +22,18 @@
       "category": "money",
       "tier": "core"
     },
+    {
+      "id": "bitcoin-core",
+      "title": "Bitcoin Core",
+      "version": "28.4",
+      "description": "Reference implementation of the Bitcoin protocol. Run a full node validating and relaying blocks on the Bitcoin network.",
+      "icon": "/assets/img/app-icons/bitcoin-core.svg",
+      "author": "Bitcoin Core contributors",
+      "dockerImage": "bitcoin/bitcoin:28.4",
+      "repoUrl": "https://github.com/bitcoin/bitcoin",
+      "category": "money",
+      "tier": "optional"
+    },
     {
       "id": "lnd",
       "title": "LND",