task: apply OS2ai colour palette as design tokens

[yepzdk]

Goal

Make the OS2ai colours the app's palette, exposed as CSS custom properties (design tokens).

Scope

• Define --color-* tokens from the hex values documented in task: source official OS2ai brand assets (logo, colours, fonts) #46 — in assets/app.css or a dedicated tokens.css.
• Apply tokens to background, text, links, and the header.
• No hard-coded hex scattered through templates/CSS — reference the tokens.

Acceptance criteria

• Documented OS2ai colours exist as CSS custom properties.
• Base layout / header use the tokens (no stray hard-coded hex).
• Passes the relevant coding-standards check (prettier for CSS via the dev profile).

Dependencies

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

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