task: adopt OS2ai-aligned typography / fonts

[yepzdk]

Goal

Use the OS2ai font family (or the closest licensed / web-safe match documented in #46) across the app.

Scope

• Load the font — self-hosted under assets/ or a documented CDN URL.
• Set base font-family, font sizes, line-height, and a headings scale (via tokens where sensible).
• Define a sensible fallback stack.

Acceptance criteria

• Base typography matches the documented OS2ai choice.
• Fonts load without console errors.
• Fallback font stack defined.

Dependencies

• Blocked by task: source official OS2ai brand assets (logo, colours, fonts) #46 (need the official font name/source).

Refs https://leantime.itkdev.dk/#/tickets/showTicket/7714