ci(frontend): concurrency optimizations (#11525)

## Changes 🏗️

Added the same concurrency optimisation to the Front-end and Fullstack
CI workflows. It will:

- Cancel in-progress runs when a new workflow starts for the same
branch/PR
- Reduce CI costs by avoiding redundant runs
- Ensure only the latest workflow runs
- Both workflows now use the same concurrency strategy to optimise CI
billing.

## Checklist 📋

### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] We will see as we push commits...

.github/workflows/platform-frontend-ci.yml

@@ -12,6 +12,10 @@ on:
       - "autogpt_platform/frontend/**"
   merge_group:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event.pull_request.number || '' }}
+  cancel-in-progress: true
+
 defaults:
   run:
     shell: bash

.github/workflows/platform-fullstack-ci.yml

@@ -12,6 +12,10 @@ on:
       - "autogpt_platform/**"
   merge_group:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event.pull_request.number || '' }}
+  cancel-in-progress: true
+
 defaults:
   run:
     shell: bash