feat(app): use the project color as the primary color of the admin UI

[slyk]

Change description

Part of discussion #431 (item 1).

The "Project Color" in Settings now becomes the primary color of the whole admin UI: buttons, links, toggles, focus rings and selections. Before, it only tinted the logo square and the favicon, because --primary and its shades (--primary-10 … --primary-190) are generated by Sass from a fixed purple at build time. For new users this is the easiest way to give the admin their own look, without touching CSS.

How it works:

• app/src/utils/project-primary-vars.ts computes --primary and every shade with CSS color-mix(), using the same mixing rules as the Sass generate-colors palette. The themes expose the colors to mix with (--primary-mix-pos / --primary-mix-neg), so light and dark mode stay correct, including auto.
• app.vue applies the variables on <body>, not on the app root, so teleported menus and dialogs (#menu-outlet, #dialog-outlet) are covered too.
• Without a project color, or in a browser without color-mix() support, nothing is set and the stock purple palette is used as before.
• A unit test pins the generated values to the Sass palette for the default purple, so the two cannot drift apart.

Screenshots and a screencast are in #431.

Type of change

• Bug fix (fixes an issue)
• New feature (adds functionality)

Related issues

Discussion #431

Checklists

Development

• Lint rules pass locally
• Application changes have been tested thoroughly
• Automated tests covering modified code pass

Security

• Security impact of change has been considered: UI only. The value comes from the existing project_color setting, which only admins can change, and is only used inside CSS custom properties.
• Code follows company security practices and guidelines

Network

• Changes to network configurations have been reviewed (not applicable)
• Any newly exposed public endpoints or data have gone through security review (not applicable)

Code review

• Pull request has a descriptive title and context useful to a reviewer. Screenshots or screencasts are attached as
  necessary
• reviewers assigned
• Pull request linked to task tracker where applicable