feat(utils): formatDate helper + sample sweep through PDF + template paths

Phase 7 — single source of truth for date display. Backed by Intl.DateTimeFormat
(no new dep — built into Node 18+ + every supported browser). Replaces 96
ad-hoc `new Date(x).toLocaleDateString('en-GB')` calls scattered across the
codebase.

src/lib/utils/format-date.ts (new):
  formatDate(value, preset?, options?)         — primary helper
  formatDateRange(start, end, options?)        — collapsed range strings
  formatRelative(value, options?)              — "3 hours ago" / "in 2 days"

  Presets (named so callers don't memorize Intl options shape):
    date.short        12 May
    date.medium       12 May 2026
    date.long         Monday, 12 May 2026
    date.iso          2026-05-12 (TZ-aware ISO date, no time)
    datetime.short    12 May 14:30
    datetime.medium   12 May 2026 14:30
    datetime.long     Monday, 12 May 2026 at 14:30 UTC
    datetime.iso      2026-05-12T14:30:00.000Z
    time              14:30

  Defensive defaults:
    - null/undefined/Invalid Date → '—' (overridable via { fallback })
    - locale defaults to en-GB (settles audit-flagged en-US/en-GB drift)
    - tz passthrough to Intl.DateTimeFormat timeZone field (any IANA name)

Sample sweep (3 sites — proves the pattern; remaining 93 sites can be
migrated opportunistically when files are touched):
  src/lib/services/expense-pdf.service.ts:608  default subheader
  src/lib/services/document-templates.ts:364   {{interest.dateFirstContact}}
  src/lib/services/document-templates.ts:374-378  {{interest.date*Signed}}

The 93 remaining sites are listed in docs/BACKLOG.md §G with the rule:
"replace as you touch the file" — gives compounding cleanup without
a single risky 90-file commit.

tests/unit/format-date.test.ts (new) — 17 tests:
  - fallback handling (null/undefined/invalid/explicit)
  - date.iso correctness in UTC + non-UTC timezones
  - datetime.iso = full ISO string
  - en-GB locale-formatted output
  - timezone respect across NY/UTC
  - time-only preset
  - Date/string/epoch ms inputs all accepted
  - formatDateRange same-year collapse, different-year keep, missing ends
  - formatRelative: just-now / minutes / hours / days / future / invalid

1315/1315 vitest green (+17 new from format-date.test.ts).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/format-date.test.ts

@@ -0,0 +1,112 @@
+import { describe, expect, it } from 'vitest';
+
+import { formatDate, formatDateRange, formatRelative } from '@/lib/utils/format-date';
+
+describe('formatDate', () => {
+  const REF = '2026-05-12T14:30:45.000Z';
+
+  it('returns fallback for null/undefined/invalid', () => {
+    expect(formatDate(null)).toBe('—');
+    expect(formatDate(undefined)).toBe('—');
+    expect(formatDate('not a date')).toBe('—');
+    expect(formatDate(NaN)).toBe('—');
+    expect(formatDate(null, 'date.medium', { fallback: 'N/A' })).toBe('N/A');
+  });
+
+  it('formats date.iso correctly in UTC', () => {
+    expect(formatDate(REF, 'date.iso', { tz: 'UTC' })).toBe('2026-05-12');
+  });
+
+  it('formats date.iso correctly in a different timezone', () => {
+    // 14:30 UTC = 00:30 next day in Sydney (UTC+10)
+    expect(formatDate(REF, 'date.iso', { tz: 'Australia/Sydney' })).toBe('2026-05-13');
+  });
+
+  it('formats datetime.iso as full ISO string', () => {
+    expect(formatDate(REF, 'datetime.iso')).toBe('2026-05-12T14:30:45.000Z');
+  });
+
+  it('formats date.medium in en-GB', () => {
+    // Intl output may vary slightly across Node versions; assert it
+    // contains the expected day + month + year and uses no "/".
+    const out = formatDate(REF, 'date.medium', { tz: 'UTC' });
+    expect(out).toMatch(/12\s/); // day
+    expect(out).toContain('2026');
+    expect(out.toLowerCase()).toContain('may');
+  });
+
+  it('respects timezone — datetime.short in different zones', () => {
+    const ny = formatDate(REF, 'datetime.short', { tz: 'America/New_York' });
+    const utc = formatDate(REF, 'datetime.short', { tz: 'UTC' });
+    expect(ny).not.toBe(utc);
+    expect(utc).toContain('14:30');
+  });
+
+  it('formats time-only correctly', () => {
+    expect(formatDate(REF, 'time', { tz: 'UTC' })).toBe('14:30');
+  });
+
+  it('accepts Date instances + epoch ms', () => {
+    const d = new Date(REF);
+    expect(formatDate(d, 'date.iso', { tz: 'UTC' })).toBe('2026-05-12');
+    expect(formatDate(d.getTime(), 'date.iso', { tz: 'UTC' })).toBe('2026-05-12');
+  });
+});
+
+describe('formatDateRange', () => {
+  it('handles missing start/end', () => {
+    expect(formatDateRange(null, null)).toBe('—');
+    expect(formatDateRange('2026-05-12', null)).toMatch(/→$/);
+    expect(formatDateRange(null, '2026-05-12')).toMatch(/^→/);
+  });
+
+  it('collapses year when start + end are in same year', () => {
+    const out = formatDateRange('2026-05-12', '2026-08-14', { tz: 'UTC' });
+    // Start should NOT include year; end should.
+    expect(out).toContain('→');
+    expect(out).toContain('2026');
+    // Crude check: there's only one "2026" in the output.
+    expect((out.match(/2026/g) ?? []).length).toBe(1);
+  });
+
+  it('keeps year on both when years differ', () => {
+    const out = formatDateRange('2025-12-20', '2026-01-05', { tz: 'UTC' });
+    expect((out.match(/202\d/g) ?? []).length).toBe(2);
+  });
+});
+
+describe('formatRelative', () => {
+  const NOW = new Date('2026-05-12T12:00:00.000Z');
+
+  it('returns "just now" for tiny deltas', () => {
+    expect(formatRelative(new Date(NOW.getTime() - 5_000), { now: NOW })).toBe('just now');
+  });
+
+  it('uses minutes for past deltas under an hour', () => {
+    const ten = new Date(NOW.getTime() - 10 * 60_000);
+    const out = formatRelative(ten, { now: NOW });
+    expect(out).toMatch(/minute/);
+  });
+
+  it('uses hours for past deltas in the same day', () => {
+    const three = new Date(NOW.getTime() - 3 * 3600_000);
+    const out = formatRelative(three, { now: NOW });
+    expect(out).toMatch(/hour/);
+  });
+
+  it('uses days for past deltas across days', () => {
+    const yesterday = new Date(NOW.getTime() - 86_400_000);
+    const out = formatRelative(yesterday, { now: NOW });
+    expect(out).toMatch(/day|yesterday/i);
+  });
+
+  it('uses future tense for future dates', () => {
+    const tomorrow = new Date(NOW.getTime() + 86_400_000);
+    const out = formatRelative(tomorrow, { now: NOW });
+    expect(out).toMatch(/day|tomorrow/i);
+  });
+
+  it('returns fallback for invalid input', () => {
+    expect(formatRelative(null)).toBe('—');
+  });
+});