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/views/settings.js

@@ -241,6 +241,12 @@ function init(ctx) {
         }
     });
 
+    $("settings-utc-timestamps").checked = state.utcTimestamps;
+    $("settings-utc-timestamps").addEventListener("change", async () => {
+        state.utcTimestamps = $("settings-utc-timestamps").checked;
+        await saveState();
+    });
+
     $("btn-main-add-wallet").addEventListener("click", ctx.showAddWalletView);
 
     $("btn-settings-add-token").addEventListener(