feat: show live build version in web UI

- /api/version returns version, commitSha, buildDate, trashRetentionDays
- CI passes COMMIT_SHA (full), BUILD_DATE and VERSION (git tag) as
  docker build args; Dockerfile bakes them as env
- sidebar footer shows version/sha linked to the Gitea commit
- admin System tab shows version, commit link, build date, trash retention

lib/api-client-react/src/generated/api.schemas.ts

@@ -9,6 +9,15 @@ export interface HealthStatus {
   status: string;
 }
 
+export interface VersionInfo {
+  version: string;
+  /** @nullable */
+  commitSha?: string | null;
+  /** @nullable */
+  buildDate?: string | null;
+  trashRetentionDays?: number;
+}
+
 export type AuthModeMode = typeof AuthModeMode[keyof typeof AuthModeMode];