fix(web): compare zones on fields Intl has always had

dateStyle/timeStyle are late additions to Intl -- Firefox shipped them in
91 -- and an implementation that does not know them ignores them and
formats the date alone. The comparison would then read New York, Chicago
and Madrid as the same zone and tick the step for a timezone that is
plainly wrong, which is the failure the check exists to catch. Silent, and
only on older browsers.

Explicit numeric fields (year/month/day/hour/minute) have been in Intl
since ECMA-402 v1, so there is nothing left to degrade to.

The options look like a stylistic choice, so a test pins them: it reads the
comparison with comments stripped -- the comment names dateStyle to explain
why it is not used -- and fails if either style option comes back or a
time field is dropped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

test/test_onboarding_checklist.py

@@ -135,6 +135,29 @@ def test_missing_timezone_leaves_the_step_open():
     assert 'data-done="0"' in step
 
 
+def test_zone_comparison_asks_for_the_time_of_day():
+    """Guard on the Intl options, which look like a stylistic choice.
+
+    dateStyle/timeStyle are late additions (Firefox shipped them in 91). An
+    implementation that does not know them ignores them and formats the date
+    alone -- which compares New York, Chicago and Madrid as equal and ticks
+    the step for a timezone that is plainly wrong. Explicit numeric fields
+    have been in Intl since ECMA-402 v1.
+    """
+    template = (PROJECT_ROOT / "web_interface" / "templates" / "v3"
+                / "partials" / "overview.html").read_text()
+    body = template[template.index("function sameZone"):]
+    body = body[:body.index("}())")]
+    # The comment above the options names dateStyle/timeStyle to explain why
+    # they are not used, so match on code only.
+    body = "\n".join(line for line in body.splitlines()
+                     if not line.lstrip().startswith("//"))
+    assert "dateStyle" not in body and "timeStyle" not in body, (
+        "zone comparison must not depend on dateStyle/timeStyle")
+    for field in ("hour:", "minute:", "year:", "month:", "day:"):
+        assert field in body, f"zone comparison dropped {field!r}"
+
+
 @pytest.mark.parametrize(
     "hardware,expected",
     [

web_interface/templates/v3/partials/overview.html

@@ -191,8 +191,18 @@
         try {
             var now = new Date();
             var stamp = function (tz) {
+                // Explicit numeric fields rather than dateStyle/timeStyle:
+                // those are late additions to Intl (Firefox shipped them in
+                // 91), and an implementation that does not know them ignores
+                // them and formats the date alone. That would compare
+                // New York, Chicago and Madrid as equal and tick the step for
+                // a timezone that is plainly wrong -- the exact failure this
+                // check exists to catch. These options have been in Intl
+                // since ECMA-402 v1.
                 return new Intl.DateTimeFormat('en-US', {
-                    timeZone: tz, dateStyle: 'short', timeStyle: 'short'
+                    timeZone: tz, year: 'numeric', month: '2-digit',
+                    day: '2-digit', hour: '2-digit', minute: '2-digit',
+                    hour12: false
                 }).format(now);
             };
             return stamp(a) === stamp(b);