fix: include timezone offset in all displayed timestamps

All isoDate() functions now output proper ISO 8601 format with timezone
offset (e.g. 2026-02-28T15:30:00-08:00) instead of bare datetime strings.
Also uses 'T' separator per ISO 8601.

closes #116

src/popup/index.html

@@ -913,6 +913,12 @@
                         />
                         <span class="text-xs text-muted">gwei</span>
                     </div>
+                    <label
+                        class="text-xs flex items-center gap-1 cursor-pointer mb-1"
+                    >
+                        <input type="checkbox" id="settings-utc-timestamps" />
+                        UTC Timestamps
+                    </label>
                 </div>
 
                 <div class="bg-well p-3 mx-1 mb-3">