refactor(MentionDropdown): receive reactive state via single 'model' prop

Svelte 5's mount() does not return prop accessors — setting
'instance.items = newValue' is a no-op. Switching to a single $state
proxy passed as 'model' lets the parent mutate fields and have the
dropdown react. The prop is named 'model' (not 'state') because the
$state rune name shadows a 'state' identifier in Svelte 5 templates.

Position class also switches from absolute to fixed so viewport-
relative DOMRect coordinates from clientRect() work when the dropdown
is mounted on document.body.

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

frontend/src/lib/components/MentionDropdown.svelte

@@ -5,11 +5,18 @@ 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);
+// The dropdown receives a single reactive state object. PersonMentionEditor
+// mutates fields on this object (model.items = ..., etc.) and Svelte's $state
+// proxy reactivity propagates the change here. This is the supported way to
+// update an imperatively-mounted Svelte 5 component — `mount` does not return
+// settable prop accessors.
+type DropdownState = {
+	items: Person[];
+	command: (item: Person) => void;
+	clientRect: (() => DOMRect | null) | null;
+};
+
+let { model }: { model: DropdownState } = $props();
 
 // 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).
@@ -19,8 +26,8 @@ let clientRect = $state<(() => DOMRect | null) | null>(null);
 let highlightedIndex = $state(0);
 
 $effect(() => {
-	// Read items to subscribe; reset index whenever the list is replaced.
-	void items;
+	// Read model.items to subscribe; reset index whenever the list is replaced.
+	void model.items;
 	highlightedIndex = 0;
 });
 
@@ -38,8 +45,9 @@ type Position = {
 const DROPDOWN_CLEARANCE_PX = 200;
 
 const position = $derived.by<Position>(() => {
-	if (!clientRect) return { top: '0px', bottom: null, left: '0px' };
-	const rect = clientRect();
+	const cr = model.clientRect;
+	if (!cr) return { top: '0px', bottom: null, left: '0px' };
+	const rect = cr();
 	if (!rect) return { top: '0px', bottom: null, left: '0px' };
 
 	// Some editors report a caret DOMRect with zero width; fall back to rect.x.
@@ -63,21 +71,21 @@ const position = $derived.by<Position>(() => {
 // ---------------------------------------------------------------------------
 
 export function onKeyDown(event: KeyboardEvent): boolean {
+	const len = model.items.length;
 	if (event.key === 'ArrowDown') {
-		highlightedIndex = (highlightedIndex + 1) % Math.max(items.length, 1);
+		highlightedIndex = (highlightedIndex + 1) % Math.max(len, 1);
 		return true;
 	}
 
 	if (event.key === 'ArrowUp') {
-		highlightedIndex =
-			(highlightedIndex - 1 + Math.max(items.length, 1)) % Math.max(items.length, 1);
+		highlightedIndex = (highlightedIndex - 1 + Math.max(len, 1)) % Math.max(len, 1);
 		return true;
 	}
 
 	if (event.key === 'Enter') {
-		const selected = items[highlightedIndex];
+		const selected = model.items[highlightedIndex];
 		if (selected) {
-			command(selected);
+			model.command(selected);
 		}
 		return true;
 	}
@@ -87,7 +95,7 @@ export function onKeyDown(event: KeyboardEvent): boolean {
 }
 
 function selectItem(item: Person) {
-	command(item);
+	model.command(item);
 }
 </script>
 
@@ -103,19 +111,19 @@ function selectItem(item: Person) {
 	unauthenticated users.
 -->
 <div
-	class="absolute z-20 w-72 overflow-hidden rounded-sm border border-line bg-surface shadow-lg"
+	class="fixed 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}
+	{#if model.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)}
+		{#each model.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',