Run the Flutter web build on pull requests

Problem

The Flutter web build is never attempted before a merge. The workflow that compiles it, client-flutter-build.yml, triggers only on workflow_dispatch and push to develop / main / argos2. The only Flutter workflow that runs on a pull request is client-flutter-ci.yml, and it stops at dependency resolution and a formatting check — it never compiles.

So a PR can turn the web build red and still show all green checks. The breakage is only discovered after it lands on develop, where it also takes the GitHub Pages deployment down with it.

Evidence

PR #731 (mqtt5_client 4.17.1 to 5.0.0) merged green on 2026-08-01 and broke the web build for five weeks. It went unnoticed until #763 fixed it on 2026-09-05. The five Deploy to GitHub Pages runs on develop between those dates were all failures; the first success afterwards is run 33997483695.

What to do

Add a pull_request trigger to client-flutter-build.yml so the build job compiles on PRs targeting develop and main.

The wrinkle: that workflow has two jobs, build and deploy. Only build may run on a pull request. The deploy job publishes to the github-pages environment and must stay restricted to push and workflow_dispatch — no PR, least of all one from a fork, should ever be able to deploy.

Acceptance

• A PR that touches flutter-client and fails to compile shows a failing check.
• The deploy job does not run on any pull_request event.
• Deploys from a push to develop / main / argos2 keep working exactly as before.