wrongmove: write VITE_MAPBOX_TOKEN to .env.production in CI (replaces broken build_args)

The previous attempt passed the Mapbox token via `--build-arg`, but
the docker-buildx plugin's KEY=VALUE list-parser mangled the value
(the rendered command was `--build-arg *=VITE_MAPBOX_TOKEN=********`,
key got lost). Inspecting `viktorbarzin/immoweb:45` confirmed
`pk.eyJ...` was nowhere in the bundle.

Switching to the idiomatic Vite path: a new `prepare-frontend-env`
commands step writes `frontend/.env.production` from the
`wrongmove-mapbox-token` Woodpecker secret. `COPY . .` in the
Dockerfile pulls the file into the build context, and Vite
auto-loads `.env.production` during `npx vite build`.

Net diff:
- `.woodpecker/frontend.yml`: new prepare step, build step now
  depends on it, dropped the build_args line.
- `frontend/Dockerfile`: dropped the ARG/ENV lines (no longer needed,
  also silences `SecretsUsedInArgOrEnv` linter warning).
- `frontend/.gitignore`: ignore `.env.production` / `.env.local` so
  the CI-written file never gets accidentally committed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/.gitignore

@@ -12,6 +12,10 @@ dist
 dist-ssr
 *.local
 
+# Vite env files written by CI (Woodpecker prepare-frontend-env step)
+.env.production
+.env.local
+
 # Editor directories and files
 .vscode/*
 !.vscode/extensions.json