a11y: decide a readable VSL link accent over background images

[Zaldaryon]

Problem

PR #236 improved text contrast over arbitrary background images, but the VSL
accent remains below the normal-text target in one worst-case stack. The
--color-vsl value #ad7639 measures about 3.15:1 for links over a white
panel background. It measures about 5.19:1 over black.

The accent is used by eight link call sites, selected borders, the active menu
marker, and the enabled toggle. Changing the variable globally is therefore a
palette decision, not a link-only contrast fix.

Decision needed

Choose one of these paths:

1. Adjust --color-vsl to a value that meets the normal-text target on the
   white worst case while preserving the intended brand appearance.
2. Keep the brand accent for non-text decoration and introduce a separate
   accessible link color for the eight text-link call sites.

Whichever path is chosen should add a contrast test for the affected link
surfaces and keep selected borders, the menu marker, and the toggle visually
coherent.

Related: #236

[Pixnop]

Shipped in #251: the accent moved to a value reading 4.85:1 on the worst stack, and every one of the eight link sites gained an underline so colour is no longer the only cue. Both are pinned in the contrast suite.