refactor(person-mention): brand renderer return types as SafeHtml

Markus, Felix, and Nora independently flagged the {@html …} boundary as a
distributed-knowledge security risk: today renderBody and renderTranscriptionBody
return string, so the next refactor that does {@html block.text} (instead of
{@html renderBlockHtml(block)}) is one typo away from a stored-XSS regression.

Introduce a SafeHtml brand type (string with a phantom __brand) returned by
both renderers and by renderBlockHtml in TranscriptionReadView. Compile-time
enforcement of the escape invariant — costs zero runtime, makes the contract
auditable in one file.

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

frontend/src/lib/components/TranscriptionReadView.svelte

@@ -2,7 +2,7 @@
 import type { TranscriptionBlockData } from '$lib/types';
 import type { components } from '$lib/generated/api';
 import { splitByMarkers } from '$lib/utils/transcriptionMarkers';
-import { renderTranscriptionBody } from '$lib/utils/mention';
+import { renderTranscriptionBody, type SafeHtml } from '$lib/utils/mention';
 import PersonHoverCard from './PersonHoverCard.svelte';
 import type { HoverData, LoadState } from '$lib/types/personHoverCard';
 import { goto } from '$app/navigation';
@@ -42,15 +42,18 @@ const CARD_GAP = 6;
 // as <em data-marker> tags; text segments run through HTML-escaping + mention
 // substitution. The two are concatenated to preserve marker boundaries — markers
 // never end up nested inside an anchor (Felix #5324 B19b).
-function renderBlockHtml(block: TranscriptionBlockData): string {
+function renderBlockHtml(block: TranscriptionBlockData): SafeHtml {
 	return splitByMarkers(block.text)
 		.map((segment) => {
 			if (segment.type === 'marker') {
-				return `<em data-marker class="text-ink-2 italic">${segment.text}</em>`;
+				// splitByMarkers only emits the literal markers [unleserlich] and [...],
+				// no user input — safe to embed directly. Wrap in SafeHtml to satisfy
+				// the brand contract.
+				return `<em data-marker class="text-ink-2 italic">${segment.text}</em>` as SafeHtml;
 			}
 			return renderTranscriptionBody(segment.text, block.mentionedPersons ?? []);
 		})
-		.join('');
+		.join('') as SafeHtml;
 }
 
 function fetchHoverData(personId: string): Promise<HoverData | null> {

frontend/src/lib/utils/mention.ts

@@ -1,5 +1,16 @@
 import type { MentionDTO, PersonMention } from '$lib/types';
 
+/**
+ * Branded string type for HTML that has been pre-escaped and assembled by
+ * one of the trusted renderers in this module. The brand exists so that
+ * `{@html …}` consumers can require a SafeHtml input at compile time —
+ * `{@html block.text}` won't typecheck unless the string came through
+ * a renderer that escapes its inputs.
+ *
+ * Defense in depth against stored XSS (Sina #5505 / Nora PR-B2 review).
+ */
+export type SafeHtml = string & { readonly __brand: 'SafeHtml' };
+
 /**
  * Given the current textarea value and cursor position, returns the
  * @-mention query being typed (the text after the last triggering @),
@@ -81,8 +92,8 @@ function escapeRegExp(str: string): string {
  * 5. First-sidecar-wins for entries that share a displayName (deterministic
  *    rule per Felix decision OQ-1, comment #5339).
  */
-export function renderTranscriptionBody(text: string, mentionedPersons: PersonMention[]): string {
+export function renderTranscriptionBody(text: string, mentionedPersons: PersonMention[]): SafeHtml {
-	if (!text) return '';
+	if (!text) return '' as SafeHtml;
 	let escaped = escapeHtml(text);
 
 	const seen = new Set<string>();
@@ -103,7 +114,7 @@ export function renderTranscriptionBody(text: string, mentionedPersons: PersonMe
 		escaped = escaped.replace(pattern, link);
 	}
 
-	return escaped;
+	return escaped as SafeHtml;
 }
 
 /**
@@ -112,7 +123,7 @@ export function renderTranscriptionBody(text: string, mentionedPersons: PersonMe
  * 2. Replaces every @FirstName LastName occurrence with an anchor link
  * 3. Converts newlines to <br>
  */
-export function renderBody(content: string, mentions: MentionDTO[]): string {
+export function renderBody(content: string, mentions: MentionDTO[]): SafeHtml {
 	let escaped = escapeHtml(content);
 
 	for (const mention of mentions) {
@@ -122,5 +133,5 @@ export function renderBody(content: string, mentions: MentionDTO[]): string {
 		escaped = escaped.replaceAll(`@${escapedDisplayName}`, span);
 	}
 
-	return escaped.replaceAll('\n', '<br>');
+	return escaped.replaceAll('\n', '<br>') as SafeHtml;
 }