fix(person-mention): textarea focus ring + 44px tap target

Leonie #5507 concerns 4 + 6:
- The textarea had outline-none and no focus indicator — broken for
  keyboard-only navigation now that the typeahead is fully keyboard-driven.
- A rows=1 textarea is ~24px tall (Merriweather + 1.625 line-height),
  below the WCAG 2.2 AA Target Size (44×44) requirement for the focused
  actionable element.

Add focus-visible ring/border in brand-mint and a min-h of 44px with
py-2.5 padding so the empty-state textarea hits the target.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -191,7 +191,7 @@ const popupOpen = $derived(query !== null);
 <div class="relative">
 	<textarea
 		{@attach attachTextarea}
-		class="w-full resize-none border-none bg-transparent font-serif text-base leading-relaxed text-ink outline-none placeholder:text-ink-3"
+		class="block min-h-[44px] w-full resize-none rounded-sm border border-transparent bg-transparent px-1 py-2.5 font-serif text-base leading-relaxed text-ink placeholder:text-ink-3 focus-visible:border-brand-mint focus-visible:ring-2 focus-visible:ring-brand-mint/40 focus-visible:outline-none"
 		rows={rows}
 		placeholder={placeholder}
 		disabled={disabled}