feat(setup): show running build version in the Setup tab bar

CI already computes <branch>-<short-sha> for image tags but never
surfaced it anywhere reachable from the app, so there was no way to
tell what's actually deployed on a booth without cross-referencing
komodo/resources.toml's TAG by hand.

Thread it through: CI passes BUILD_VERSION as a Docker build-arg,
the Dockerfile captures it as a runtime env var, GET /api/version
(gated by the existing site:read permission) exposes it, and the
Setup page's tab bar shows it right-aligned, muted, absent entirely
on a local/dev build with no CI-supplied value.

Claude-Session: https://claude.ai/code/session_01FWncR69HgGPuei1dLrW3cU

.gitea/workflows/build-images.yml

@@ -92,6 +92,8 @@ jobs:
           context: .
           file: apps/server/Dockerfile
           push: true
+          build-args: |
+            BUILD_VERSION=${{ steps.meta.outputs.branch }}-${{ steps.meta.outputs.sha }}
           tags: |
             ${{ env.REGISTRY }}/parking-server:${{ steps.meta.outputs.branch }}
             ${{ env.REGISTRY }}/parking-server:${{ steps.meta.outputs.branch }}-${{ steps.meta.outputs.sha }}