fix(ci): grant contents write permission to gh-pages deploy

[ppettitau]

Problem

The deploy job failed after merging the Angular 22 upgrade with:

remote: Permission to eresearchqut/angular-password-strength-meter.git denied to github-actions[bot].
... error: 403

Cause

The default GITHUB_TOKEN is read-only, so JamesIves/github-pages-deploy-action cannot push to the gh-pages branch.

Fix

Grant contents: write permission to the deploy job.

Note: this only works if the repo's Settings → Actions → General → Workflow permissions allows workflows to request write (i.e. not forced read-only). If the deploy still 403s, that setting must be enabled.