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/PersonMentionEditor.svelte

@@ -103,6 +103,9 @@ onMount(() => {
 			}),
 			CustomMention.configure({
 				renderHTML({ node }) {
+					// Underline color matches the read-mode .person-mention rule
+					// (ink at ~50% alpha) — brand-mint on white fails WCAG 1.4.11
+					// Non-Text Contrast (≈1.7:1, needs 3:1). Leonie #5621.
 					return [
 						'span',
 						{
@@ -110,7 +113,7 @@ onMount(() => {
 							'data-person-id': node.attrs.personId,
 							'data-display-name': node.attrs.displayName,
 							class:
-								'mention-token underline decoration-brand-mint underline-offset-2 text-brand-navy font-medium'
+								'mention-token underline decoration-ink/50 underline-offset-2 text-brand-navy font-medium'
 						},
 						`@${node.attrs.displayName}`
 					];