feat(integration): add docs/visio logos, theme.css, and nginx updates

Add SVG logos for docs and visio apps. Update drive, mail, people logos.
Add theme.css served at /api/v2/theme.css for La Suite frontend theming.
Update nginx to serve theme endpoint and tighten cache headers.
Update Dockerfile to include new static assets.

Dockerfile

@@ -1,7 +1,7 @@
 # ── Stage 1: build lagaufre.js from suitenumerique/integration source ──────────
 # Build context must be the sunbeam/ root so we can reach both
 # integration/packages/widgets/ and integration-service/.
-FROM node:22-alpine AS widget-build
+FROM --platform=$BUILDPLATFORM node:22-alpine AS widget-build
 WORKDIR /src
 COPY integration/packages/widgets/ .
 RUN npm ci && npm run build
@@ -23,5 +23,8 @@ COPY --from=widget-build /src/dist/lagaufre.js /usr/share/nginx/html/widget/laga
 # Derives its origin from the script URL at runtime; no DOMAIN_SUFFIX baked in.
 COPY integration-service/gaufre.js /usr/share/nginx/html/gaufre.js
 
+# Runtime brand CSS variable overrides for the estudio theme.
+COPY integration-service/theme.css /usr/share/nginx/html/theme.css
+
 # Nginx config — only services.json is mounted at runtime (from ConfigMap).
 COPY integration-service/nginx.conf /etc/nginx/conf.d/default.conf