fix(ui): show version+SHA+build time in footer; fix env not reaching runtime image

PUBLIC_BUILD_VERSION and PUBLIC_BUILD_COMMIT were set only in the builder
stage ENV — they were never re-declared in the runtime stage, so the Node
server started with them undefined and the badge always showed 'dev'.

Fix: re-declare all three ARGs after the second FROM and set runtime ENVs.
Add PUBLIC_BUILD_TIME (ISO timestamp from gitea.event.head_commit.timestamp)
injected via build-arg in release.yaml. Badge now shows e.g.:
  v2.3.9+abc1234 · 28 Mar 2026 14:30 UTC

.gitea/workflows/release.yaml

@@ -218,6 +218,7 @@ jobs:
           build-args: |
             BUILD_VERSION=${{ steps.meta.outputs.version }}
             BUILD_COMMIT=${{ gitea.sha }}
+            BUILD_TIME=${{ gitea.event.head_commit.timestamp }}
           cache-from: type=registry,ref=${{ secrets.DOCKER_USER }}/libnovel-ui:latest
           cache-to: type=inline