refactor(frontend): delete orphaned personMention.ts after Tiptap migration

The textarea-era detectPersonMention helper has no production callers since
the suggestion plugin's char: '@' mechanism replaced it. Per "Dead code is
deleted, not commented out", remove the source file and its spec — the spec
was running but tested a function nobody calls.

Felix #5615 blocker.

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

frontend/src/lib/utils/personMention.spec.ts

@@ -1,65 +0,0 @@
-import { describe, it, expect } from 'vitest';
-import { detectPersonMention } from './personMention';
-
-describe('detectPersonMention', () => {
-	it('returns null when text has no @', () => {
-		expect(detectPersonMention('hello world', 11)).toBeNull();
-	});
-
-	it('returns null when @ is preceded by a non-whitespace character (email pattern)', () => {
-		expect(detectPersonMention('user@example', 12)).toBeNull();
-	});
-
-	it('returns query for @ at the very start of string', () => {
-		expect(detectPersonMention('@Aug', 4)).toBe('Aug');
-	});
-
-	it('returns empty string immediately after @', () => {
-		expect(detectPersonMention('@', 1)).toBe('');
-	});
-
-	it('returns single-word query', () => {
-		expect(detectPersonMention('hi @Auguste', 11)).toBe('Auguste');
-	});
-
-	it('keeps the trigger active when the query has a trailing space', () => {
-		expect(detectPersonMention('hi @Auguste ', 12)).toBe('Auguste ');
-	});
-
-	it('returns multi-word query (spaces allowed)', () => {
-		expect(detectPersonMention('hi @Auguste Raddatz', 19)).toBe('Auguste Raddatz');
-	});
-
-	it('returns single-character query', () => {
-		expect(detectPersonMention('@M', 2)).toBe('M');
-	});
-
-	it('returns null when the query crosses a newline', () => {
-		expect(detectPersonMention('@Aug\nfoo', 8)).toBeNull();
-	});
-
-	it('returns null when a second @ appears in the query (next mention starts)', () => {
-		expect(detectPersonMention('@Aug@bar', 8)).toBeNull();
-	});
-
-	it('uses the most recent @ when separated by whitespace', () => {
-		// '@Aug @Bert' with cursor at end — the second @ is the trigger.
-		expect(detectPersonMention('@Aug @Bert', 10)).toBe('Bert');
-	});
-
-	it('returns the query when the cursor sits exactly at a newline boundary', () => {
-		// '@Aug\nfoo' with cursor at index 4 — right at the newline before it
-		// is consumed. The query is still 'Aug' because nothing past the cursor
-		// counts.
-		expect(detectPersonMention('@Aug\nfoo', 4)).toBe('Aug');
-	});
-
-	it('returns null when cursor is before the @', () => {
-		expect(detectPersonMention('@Hans', 0)).toBeNull();
-	});
-
-	it('uses the most recent @ in the text', () => {
-		// cursor is just after the second @ + a few chars
-		expect(detectPersonMention('hi @Anna and @Bert', 18)).toBe('Bert');
-	});
-});

frontend/src/lib/utils/personMention.ts

@@ -1,23 +0,0 @@
-/**
- * Given the current textarea value and cursor position, returns the
- * @-person-mention query being typed (the text after the last triggering @),
- * or null if no person-mention is active.
- *
- * Rules — distinct from comment-mentions in `mention.ts`:
- * - @ must be at the start of the string or preceded by whitespace
- * - The query may contain spaces (historical persons commonly have multi-word
- *   display names — "Auguste Raddatz", "Maria von Müller-Schultz")
- * - The query stops at a newline or at a second @ (the next mention starts)
- */
-export function detectPersonMention(text: string, cursorPos: number): string | null {
-	const before = text.slice(0, cursorPos);
-	const atIndex = before.lastIndexOf('@');
-	if (atIndex === -1) return null;
-
-	if (atIndex > 0 && !/\s/.test(before[atIndex - 1])) return null;
-
-	const query = before.slice(atIndex + 1);
-	if (query.includes('\n') || query.includes('@')) return null;
-
-	return query;
-}