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

apps/web/src/api.ts

@@ -253,6 +253,15 @@ export async function fetchBackupStatus(): Promise<BackupStatus> {
   return apiFetch("/api/backup/status");
 }
 
+export interface VersionInfo {
+  /** "<branch>-<short-sha>" baked in at image build time; null on a local/dev build. */
+  buildVersion: string | null;
+}
+
+export async function fetchVersion(): Promise<VersionInfo> {
+  return apiFetch("/api/version");
+}
+
 export interface BackupConfigPatch {
   /** "" clears the target. Omit a field to leave it unchanged; null resets retention to default. */
   targetDir?: string | null;