[S4] First-login onboarding tutorial

A slider-style onboarding modal shown once per user on their first login, with role-specific content for Admins, Editors, Members, and Viewers.

Why this matters

New users arriving in GanttForce without context have to figure out the interface on their own. A short, tailored walkthrough on first login introduces the parts of the product most relevant to their role, reducing time-to-first-value and the number of basic questions that reach admins.

How it works

On the user's first login the platform shows a slider modal with a small set of slides. Which slides appear is determined automatically by the user's role — each role (Admin, Editor, Member, Viewer) has its own set, and some slides are shared across roles so they are written once and reused. Slides are stored as HTML pages, making them easy to edit and style without a dedicated admin UI. Once dismissed, the tutorial is not shown again unless explicitly reopened.