Fix back navigation from filter views to cards (#2725)

* test: update sign_in_as system test helper to be more generic

and support fixture users with more than one board

* test: backfill a test for the card "back link"

when coming from a board filter page

* Fix back navigation to filter views from cards

When navigating from a filter view to a card, the back link now shows
the filter's label and navigates back to the filter URL instead of the
card's board.

The turbo-navigation Stimulus controller stores the page title alongside
the referrer URL in sessionStorage. On the card show page, a
referrerBackLink target rewrites the back link's href and label from the
stored values. This is scoped only to the card show page to avoid
navigation loops in multi-level deep pages.

* Add referrer allowlist to prevent stale referrer rewrites

The back link is only rewritten when the stored referrer's path matches
an allowlist of paths passed via prefer_referrer. This prevents stale
referrers from unrelated pages (e.g., settings) from overriding the
card's back link.

* Move back link navigation tests to dedicated system test file

Extract back link tests from smoke_test.rb into their own file since
they test specific JavaScript behavior that needs regression coverage.
Move sign_in_as helper to ApplicationSystemTestCase for reuse.

* Move markdown paste tests to dedicated system test file

test/application_system_test_case.rb

@@ -28,4 +28,10 @@ class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
   else
     driven_by :chrome_headless, screen_size: [ 1200, 1000 ]
   end
+
+  private
+    def sign_in_as(user)
+      visit session_transfer_url(user.identity.transfer_id, script_name: nil)
+      assert_current_path root_path
+    end
 end