Feat: upgrade dhtmlx-gantt to v10

[Rom1-B]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It replace Bump dhtmlx-gantt from 9.1.4 to 10.0.0 #219
• Upgrades the bundled dhtmlx-gantt library from v9 to v10.
• The community edition of v10 drops the marker and undo plugins used by the Gantt view, so the today-column highlight and the rollback of a rejected drag-and-drop are now handled by the plugin itself instead of relying on those plugins.
• Adds a permanent Playwright e2e suite for the Gantt view to catch regressions on this upgrade going forward, covering rendering, the today-column highlight, and drag and drop reparenting with rollback on a server-rejected change.
• Adds a dedicated CI workflow to run these tests against a real GLPI instance, since they need a browser and a database the shared plugin CI does not provide.

Screenshots (if appropriate):