build(frontend-builder): bump default PROTOC_GEN_ES_VERSION to 2.12.0

The 2.4.0 default crashed at runtime with
  "protoc-gen-es: Cannot read properties of undefined (reading 'length')"
when buf v1.55 invoked it. 2.12.x is current and has no such issue.

KollectAI-ETL's CI no longer relies on the image's globally-installed
plugin (the proto-gen workflow now does `pnpm install` in proto/ and
runs `pnpm exec buf generate` to use the lockfile-pinned version), so
this is mainly housekeeping for ad-hoc use inside the container.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ci/frontend-builder/Dockerfile

@@ -18,9 +18,11 @@ FROM node:${NODE_MAJOR}-bookworm-slim
 
 ARG PNPM_VERSION=10.15.0
 ARG BUF_VERSION=1.55.0
-# Pin protoc-gen-es to a known-good major. Bump in lockstep with the
-# frontend's @bufbuild/protobuf runtime version in package.json.
-ARG PROTOC_GEN_ES_VERSION=2.4.0
+# Pin protoc-gen-es to a known-good version. The KollectAI-ETL CI
+# workflow doesn't actually use this global install (it does
+# `pnpm install` from proto/ to pick up the lockfile-pinned version)
+# but we still bake it for ad-hoc use inside the container.
+ARG PROTOC_GEN_ES_VERSION=2.12.0
 ARG ORAS_VERSION=1.2.2
 
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