a11y: re-evaluate .from-value / .to-value contrast across skins

[JohnMcLear]

Issue

`static/css/comment.css` styles the suggestion value spans with:

```css
.suggestion-display .from-value, .suggestion-display .to-value {
opacity: .8;
font-style: italic;
}
.suggestion-create .from-value {
display: block;
opacity: .8;
font-style: italic;
margin: 5px 0;
}
```

The `opacity: .8` reduces contrast against the underlying background. On the default `colibris` skin (white card / dark text) this still passes WCAG 2.1 AA (≥4.5:1), but on darker / branded skins or themed deployments it can drop below threshold.

What to do

1. Audit current contrast ratio across the bundled skins and the most common community skins (e.g. dark mode addons).
2. Replace `opacity: .8` with an explicit color token derived from the skin's text color, so each skin can override it without relying on alpha-blending against an unknown background.
3. Add a Playwright + axe-core check (or a simple `getComputedStyle` assertion in a frontend spec) that verifies the value text contrast meets AA on the default skin so future regressions get caught.

Scope

• `static/css/comment.css`
• Possibly per-skin overrides
• Frontend test under `static/tests/frontend-new/specs/`