Split return date nav into separate outbound/return rows; show earlier tube option on inbound

For return journeys, replace the single combined date navigation row with two
separate rows so outbound and return dates can be adjusted independently.

For inbound underground options, show one service before the earliest catchable
(as an "aim for this" option) rather than the next service after it, which
often arrived too late to connect with the GWR train.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -681,6 +681,32 @@ def _results(station_crs, slug, travel_date, journey_type, return_date):
         next_date,
         **{**common_url_args, "return_date": next_return_date},
     )
+    prev_outbound_url = _results_url(
+        station_crs, slug, prev_date, **common_url_args
+    )
+    next_outbound_url = _results_url(
+        station_crs, slug, next_date, **common_url_args
+    )
+    prev_return_url = (
+        _results_url(
+            station_crs,
+            slug,
+            travel_date,
+            **{**common_url_args, "return_date": prev_return_date},
+        )
+        if return_date
+        else None
+    )
+    next_return_url = (
+        _results_url(
+            station_crs,
+            slug,
+            travel_date,
+            **{**common_url_args, "return_date": next_return_date},
+        )
+        if return_date
+        else None
+    )
     destination_links = [
         (
             destination_slug,
@@ -775,6 +801,10 @@ def _results(station_crs, slug, travel_date, journey_type, return_date):
         next_date=next_date,
         prev_results_url=prev_results_url,
         next_results_url=next_results_url,
+        prev_outbound_url=prev_outbound_url,
+        next_outbound_url=next_outbound_url,
+        prev_return_url=prev_return_url,
+        next_return_url=next_return_url,
         destination_links=destination_links,
         results_base_url=results_base_url,
         travel_date_display=travel_date_display,