feat(forge): add GitLab support

[afonsojramos]

Adds GitLab (Cloud and Self-Managed) as a forge behind the existing ForgeAdapter interface, with no forge-specific branching outside src/renderer/utils/forges/gitlab/. GitLab has no /notifications endpoint, so the adapter is built on the To-Do API — and because a to-do embeds its full target object, enrichNotifications is omitted entirely and one list call carries everything the detailed-notifications setting needs. GitLab also has no read/unread axis (only pending/done), so capabilities.markAsDone reports false and mark-as-read maps onto POST /todos/:id/mark_as_done, keeping the row to a single button.

Worth flagging for review: clearing a to-do needs the broad api scope on legacy tokens (or the Todo resource on GitLab 18+ fine-grained tokens), and To-Do items are narrower than GitHub notifications — both are stated on the login screen. Every API behaviour was verified against live gitlab.com rather than taken from the docs, and the feature was exercised end-to-end in the app: login, render, mark-as-read round-trip, and logout.

Closes #237

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
95.3% Coverage on New Code
2.1% Duplication on New Code

See analysis details on SonarQube Cloud

[setchy]

Great work @afonsojramos - our forges continue to go from strength to strength.

I don't have a GitLab handy to test on, but lgtm