Add a working-date selector to the instructor home page

Everything on the home page — availability counts, live mode, the
calls editor, and list generation — now targets a selectable working
date instead of being fixed to today, with quick links to upcoming
scheduled class days. Printing tomorrow's list a day in advance is
just switching the date and generating; other days' data is
untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_instructor_ui.py

@@ -69,7 +69,7 @@ def test_regenerate_replaces_pending_and_labels_change(instructor):
 
     instructor.post(f"/instructor/day/{TODAY}/generate", data={"n": "12"})
     page = instructor.get("/instructor/").get_data(as_text=True)
-    assert "View today's list" in page
+    assert f"View the list for {TODAY}" in page
     assert "Regenerate list" in page
 
     # Regenerating replaces the unused list outright.
@@ -95,6 +95,26 @@ def test_regenerate_replaces_pending_and_labels_change(instructor):
     assert "already have recorded outcomes" in page
 
 
+def test_home_working_date_override(instructor):
+    tomorrow = (
+        datetime.date.today() + datetime.timedelta(days=1)
+    ).isoformat()
+    page = instructor.get(f"/instructor/?date={tomorrow}").get_data(as_text=True)
+    assert f"available on {tomorrow}" in page
+    assert f"Generate list for {tomorrow}" in page
+    assert "back to today" in page
+
+    # Generate tomorrow's list a day in advance; today stays untouched.
+    resp = instructor.post(
+        f"/instructor/day/{tomorrow}/generate",
+        data={"n": "5"},
+        follow_redirects=True,
+    )
+    assert resp.get_data(as_text=True).count("<tr>") == 6
+    page = instructor.get(f"/instructor/day/{TODAY}/print").get_data(as_text=True)
+    assert "No call list has been generated" in page
+
+
 def test_add_call_after_the_fact(instructor):
     page = instructor.get(f"/instructor/day/{TODAY}").get_data(as_text=True)
     student_id = re.search(r'<option value="(\d+)">', page).group(1)