fix: #3619933 Default admin high contrast mode and focus ring meet WCAG

Rework the default_admin high-contrast (Increase contrast) mode so it preserves the dark surface palette and dark/light hierarchy instead of forcing --admin-color-bg to #fff or #000. The high-contrast block now lives in a single @media (forced-colors: none) rule using light-dark() values with no .gin--dark-mode descendant.

Set solid light-dark() focus-ring colors so they reach WCAG 3:1 in both modes. Convert the gin/green/claro/orange focus presets and the link hover in dark mode to a new --admin-color-accent-fg token computed with oklch() so accent values stay hue-preserved and readable.

Add functional and JavaScript tests covering light and dark contrast for accent presets, high-contrast tokens, focus rings, and interactive states. Add a Playwright spec with an axe-core contrast matrix. Update the high-contrast setting description.

Closes #3619933