feat: timezone setting — display timestamps in selected local timezone

Add a Display section to the settings modal with a timezone dropdown.
Selection is persisted to localStorage and applied to all timestamps via
fmtDate (date-only) and fmtDateFull (date + time + TZ abbreviation, e.g.
"Mar 28, 2026, 2:48 PM EDT"). Changing the timezone live-re-renders the
current page. Defaults to UTC.

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

tests/helpers.test.js

@@ -88,24 +88,36 @@ describe('fmtDate', () => {
 
 // ── fmtDateFull() ─────────────────────────────────────────────────────────────
 
-function fmtDateFull(d) {
+function fmtDateFull(d, tz = 'UTC') {
   if (!d) return '—'
   try {
     return new Date(d).toLocaleString('en-US', {
       year: 'numeric', month: 'short', day: 'numeric',
       hour: '2-digit', minute: '2-digit',
+      timeZone: tz, timeZoneName: 'short',
     })
   } catch (e) { return d }
 }
 
 describe('fmtDateFull', () => {
   it('includes date and time components', () => {
-    const result = fmtDateFull('2024-03-15T14:30:00')
+    const result = fmtDateFull('2024-03-15T14:30:00Z')
     expect(result).toMatch(/Mar/)
     expect(result).toMatch(/2024/)
     expect(result).toMatch(/\d{1,2}:\d{2}/)
   })
 
+  it('includes the timezone abbreviation', () => {
+    expect(fmtDateFull('2024-03-15T14:30:00Z', 'UTC')).toMatch(/UTC/)
+  })
+
+  it('converts to the given timezone', () => {
+    // 2024-03-15 18:30 UTC = 2024-03-15 14:30 EDT (UTC-4 in March)
+    const result = fmtDateFull('2024-03-15T18:30:00Z', 'America/New_York')
+    expect(result).toMatch(/2:30/)
+    expect(result).toMatch(/EDT/)
+  })
+
   it('returns — for null', () => {
     expect(fmtDateFull(null)).toBe('—')
   })