feat(frontend): add MentionDropdown — Tiptap suggestion-compatible person dropdown

Replaces PersonMentionEditor's inline popup for the Tiptap migration.
Mounted imperatively to document.body by the suggestion plugin's render()
lifecycle. Supports flip-upward strategy when viewport space is tight
(Leonie #5602 mobile keyboard concern). 44px touch targets, WCAG accessible.

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

frontend/src/lib/components/MentionDropdown.svelte

@@ -0,0 +1,143 @@
+<script lang="ts">
+import type { components } from '$lib/generated/api';
+import { formatLifeDateRange } from '$lib/utils/personLifeDates';
+import { m } from '$lib/paraglide/messages.js';
+
+type Person = components['schemas']['Person'];
+
+// All reactive state is driven externally by the Tiptap suggestion plugin.
+// The parent writes to these after mount() via the exported bindings.
+let items = $state<Person[]>([]);
+let command = $state<(item: Person) => void>(() => {});
+let clientRect = $state<(() => DOMRect | null) | null>(null);
+
+// highlightedIndex must be both writable (keyboard handler mutates it) and
+// reset when `items` changes (so it never points past the end of a new list).
+// A pure $derived is read-only and cannot serve both needs, so $state + $effect
+// is the correct pattern here. The autofixer suggestion to use $derived does not
+// apply: the mutation in onKeyDown is not a derivation.
+let highlightedIndex = $state(0);
+
+$effect(() => {
+	// Read items to subscribe; reset index whenever the list is replaced.
+	void items;
+	highlightedIndex = 0;
+});
+
+// ---------------------------------------------------------------------------
+// Positioning — flip strategy: open upward when there is not enough room
+// below the cursor to show the dropdown without clipping the viewport.
+// ---------------------------------------------------------------------------
+
+type Position = {
+	top: string | null;
+	bottom: string | null;
+	left: string;
+};
+
+const DROPDOWN_CLEARANCE_PX = 200;
+
+const position = $derived.by<Position>(() => {
+	if (!clientRect) return { top: '0px', bottom: null, left: '0px' };
+	const rect = clientRect();
+	if (!rect) return { top: '0px', bottom: null, left: '0px' };
+
+	// Some editors report a caret DOMRect with zero width; fall back to rect.x.
+	const left = `${rect.width === 0 ? rect.x : rect.left}px`;
+
+	if (window.innerHeight - rect.bottom < DROPDOWN_CLEARANCE_PX) {
+		// Not enough space below — anchor bottom of dropdown to top of caret.
+		return {
+			top: null,
+			bottom: `${window.innerHeight - rect.top}px`,
+			left
+		};
+	}
+
+	return { top: `${rect.bottom}px`, bottom: null, left };
+});
+
+// ---------------------------------------------------------------------------
+// Keyboard handler — exported so Tiptap's render() can forward events.
+// Returns true when the event is consumed (prevents the editor's default).
+// ---------------------------------------------------------------------------
+
+export function onKeyDown(event: KeyboardEvent): boolean {
+	if (event.key === 'ArrowDown') {
+		highlightedIndex = (highlightedIndex + 1) % Math.max(items.length, 1);
+		return true;
+	}
+
+	if (event.key === 'ArrowUp') {
+		highlightedIndex =
+			(highlightedIndex - 1 + Math.max(items.length, 1)) % Math.max(items.length, 1);
+		return true;
+	}
+
+	if (event.key === 'Enter') {
+		const selected = items[highlightedIndex];
+		if (selected) {
+			command(selected);
+		}
+		return true;
+	}
+
+	// Escape: let the suggestion plugin handle it (return false = not consumed).
+	return false;
+}
+
+function selectItem(item: Person) {
+	command(item);
+}
+</script>
+
+<!--
+	Mounted imperatively to document.body by the Tiptap suggestion plugin.
+	Positioned absolutely relative to the viewport using inline styles derived
+	from the Tiptap clientRect() callback.
+
+	SECURITY: This component receives pre-filtered Person[] items from the
+	parent — it does NOT fetch. The parent's fetch relies on the SvelteKit Vite
+	proxy injecting the auth_token cookie as the Authorization header.
+	Mounted in transcribe mode behind WRITE_ALL — never reachable to
+	unauthenticated users.
+-->
+<div
+	class="absolute z-20 w-72 overflow-hidden rounded-sm border border-line bg-surface shadow-lg"
+	role="listbox"
+	aria-label={m.person_mention_btn_label()}
+	style:top={position.top}
+	style:bottom={position.bottom}
+	style:left={position.left}
+>
+	{#if items.length === 0}
+		<p class="px-3 py-2.5 font-sans text-sm text-ink-3">
+			{m.person_mention_popup_empty()}
+		</p>
+	{:else}
+		{#each items as person, i (person.id)}
+			<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'
+				]}
+				role="option"
+				aria-selected={i === highlightedIndex}
+				data-test-person-id={person.id}
+				tabindex="-1"
+				onmousedown={(e) => {
+					// Prevent blur on the editor before the selection fires.
+					e.preventDefault();
+					selectItem(person);
+				}}
+			>
+				<span class="truncate font-serif text-base text-ink">{person.displayName}</span>
+				{#if formatLifeDateRange(person.birthYear, person.deathYear)}
+					<span class="truncate font-sans text-xs text-ink-3">
+						{formatLifeDateRange(person.birthYear, person.deathYear)}
+					</span>
+				{/if}
+			</div>
+		{/each}
+	{/if}
+</div>