fix(docker): build the frontend stage on Node 26

.nvmrc and frontend/package.json engines moved to Node 26 / npm 11 and the
lockfile is now written by npm 11, but the Dockerfile's frontend stage stayed
on node:22-alpine. npm 10 rejects that lockfile ("Missing: [email protected] from
lock file"), so `npm ci` fails and the image no longer builds.

Dockerfile

@@ -1,7 +1,7 @@
 # ========================================================
 # Stage: Frontend (Vite)
 # ========================================================
-FROM --platform=$BUILDPLATFORM node:22-alpine AS frontend
+FROM --platform=$BUILDPLATFORM node:26-alpine AS frontend
 WORKDIR /src/frontend
 COPY frontend/package.json frontend/package-lock.json ./
 RUN npm ci