refactor(lib): consolidate generic infra into src/lib

Move domain-agnostic, feature-free helpers out of the flat utils/ and store/
roots into src/lib/ (plan M4, §3 lib/ layer):
- lib/format/: formatBytes, formatClockTime, formatDuration, formatHumanDuration,
  relativeTime (pure format/date/byte/clock helpers)
- lib/i18n.ts: i18next bootstrap (global app infra)
- lib/util/: sanitizeHtml, platform, dedupeById, safeStorage (pure helpers +
  the zustand storage adapter)

playbackScheduleFormat stays in utils/format (runtime usePlayerStore dep =
audio-core coupling, not generic). formatClockTime keeps a type-only
@/store/authStoreTypes import (erased, no runtime inversion — accepted per the
deviceSync precedent).

Pure move via deep @/lib/* specifiers (no barrel → no mock-collapse surface).
tsc 0, lint 0/0, full suite 319 files / 2353 tests green.

Tooling note: lib_move.py regex extended to also rewrite side-effect imports
(import './i18n') and vi.importActual paths — both were silent gaps.

src/lib/format/relativeTime.test.ts

@@ -0,0 +1,21 @@
+import { describe, it, expect } from 'vitest';
+import { formatRelativeTime } from '@/lib/format/relativeTime';
+
+describe('formatRelativeTime', () => {
+  const SECOND = 1000;
+  const MINUTE = 60 * SECOND;
+  const HOUR = 60 * MINUTE;
+  const DAY = 24 * HOUR;
+
+  it('picks the largest sensible unit for past dates (en)', () => {
+    expect(formatRelativeTime(new Date(Date.now() - 5 * MINUTE), 'en')).toBe('5 minutes ago');
+    expect(formatRelativeTime(new Date(Date.now() - 3 * HOUR), 'en')).toBe('3 hours ago');
+    expect(formatRelativeTime(new Date(Date.now() - 3 * DAY), 'en')).toBe('3 days ago');
+    expect(formatRelativeTime(new Date(Date.now() - 14 * DAY), 'en')).toBe('2 weeks ago');
+  });
+
+  it('handles future dates and respects the locale', () => {
+    expect(formatRelativeTime(new Date(Date.now() + 2 * DAY), 'en')).toBe('in 2 days');
+    expect(formatRelativeTime(new Date(Date.now() - 3 * DAY), 'de')).toBe('vor 3 Tagen');
+  });
+});