fix(person-mention): WCAG 1.4.11 contrast for mention pill and dropdown ring

Two non-text-contrast failures, both flagged by Leonie #5621:

1. PersonMentionEditor mention pill: decoration-brand-mint (#A6DAD8) on
   white is ≈1.7:1 — fails the 3:1 minimum for meaningful UI indicators.
   Switch to decoration-ink/50, which matches the read-mode .person-mention
   rule (≈6.4:1) and keeps a unified underline language across modes.

2. MentionDropdown highlighted-row ring: ring-brand-mint on bg-brand-mint/20
   is ≈2.5:1 — same failure class. Switch to ring-brand-navy (≈14.5:1
   against the highlight background) so keyboard-driven selection has a
   clearly visible indicator.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/MentionDropdown.svelte

@@ -127,7 +127,10 @@ function selectItem(item: Person) {
 			<div
 				class={[
 					'flex min-h-[44px] cursor-pointer flex-col gap-1 px-3 py-2.5 text-left hover:bg-canvas',
-					i === highlightedIndex && 'bg-brand-mint/20 ring-2 ring-brand-mint ring-inset'
+					// brand-mint ring (≈2.5:1 on white) fails WCAG 1.4.11 Non-Text
+					// Contrast for a meaningful keyboard-highlight indicator. brand-navy
+					// gives ~14.5:1 against the bg-brand-mint/20 row. Leonie #5621.
+					i === highlightedIndex && 'bg-brand-mint/20 ring-2 ring-brand-navy ring-inset'
 				]}
 				role="option"
 				aria-selected={i === highlightedIndex}