Fix contrast on the "Open GitHub issue" button

[screen-gd]

Problem

In light mode, the "Open GitHub issue" button in the site notice (components/SiteNotice.tsx) shows black text on bright blue. The contrast is too low to read comfortably. The button also looks unlike other buttons on the site, and "Close notice" next to it uses a different style again.

Acceptance criteria

• The button text meets WCAG AA contrast (4.5:1) in light and dark mode.
• The button uses the site's shared button styles.
• "Open GitHub issue" and "Close notice" look like a matching pair of primary and secondary actions.

[screen-gd]

This was generated by AI during triage.

Verified as implemented on main at 406f0f1.

Both notice actions use the shared Button component, with a primary issue action and a secondary dismiss action: https://github.com/screen-gd/Col/blob/406f0f1/components/SiteNotice.tsx

The app frame uses the library-page-shell theme. Its notice CSS overrides the blue utility classes with neutral theme colors: https://github.com/screen-gd/Col/blob/406f0f1/app/globals.css#L1698

Calculated primary button text contrast is 16.01:1 in light mode and 15.46:1 in dark mode, exceeding the requested 4.5:1. The production build passed. This review used source inspection and color calculations, without live browser testing. Closing as completed.