feat(ui): show build version in footer; enable watchtower for caddy

- Footer now displays build version tag + short commit SHA (from
  PUBLIC_BUILD_VERSION / PUBLIC_BUILD_COMMIT env vars baked in at
  image build time); falls back to 'dev' in local builds
- docker-compose.yml: add watchtower label to caddy service so it
  auto-updates alongside backend/ui/runner on new image pushes
- homelab/docker-compose.yml: use locally-built kokoro-fastapi:latest
  image (consistent with actual homelab setup)

homelab/docker-compose.yml

@@ -394,7 +394,7 @@ services:
   # Voices match existing IDs: af_bella, af_sky, af_heart, etc.
   # The runner reaches it at http://kokoro-fastapi:8880 via the Docker network.
   kokoro-fastapi:
-    image: ghcr.io/remsky/kokoro-fastapi-gpu:latest
+    image: kokoro-fastapi:latest
     restart: unless-stopped
     deploy:
       resources: