Improve return date UX on search form

Disable the return date input until Return journey type is selected.
Clicking anywhere in the return date group auto-selects Return and
enables the field. The return date min is kept in sync with the
outbound date, bumping the value forward if it would otherwise fall
before the outbound date.

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

templates/base.html

@@ -196,6 +196,7 @@
     /* Form groups */
     .form-group    { margin-bottom: 1.2rem; }
     .form-group-lg { margin-bottom: 1.5rem; }
+    #return-date-group:has(input:disabled) { cursor: pointer; }
 
     /* Buttons */
     .btn-primary {