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>

css/app.css

@@ -665,6 +665,9 @@ select:focus { border-color: var(--accent); }
   margin-bottom: 8px;
 }
 .settings-desc { font-size: 12px; color: var(--text2); margin: 0 0 14px; line-height: 1.6; }
+.settings-row { display: flex; align-items: center; gap: 12px; }
+.settings-label { font-size: 13px; color: var(--text2); white-space: nowrap; min-width: 80px; }
+.settings-select { flex: 1; }
 .import-row { display: flex; gap: 10px; align-items: center; }
 .import-file-input { flex: 1; }