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

app/javascript/controllers/turbo_navigation_controller.js

@@ -1,8 +1,12 @@
 import { Controller } from "@hotwired/stimulus"
 
 export default class extends Controller {
+  static values = { label: String }
+  static targets = [ "referrerBackLink" ]
+
   rememberLocation() {
     sessionStorage.setItem("referrerUrl", window.location.href)
+    sessionStorage.setItem("referrerLabel", this.labelValue)
   }
 
   backIfSamePath(event) {
@@ -16,6 +20,19 @@ export default class extends Controller {
       Turbo.visit(this.#referrerUrl)
     }
   }
+
+  referrerBackLinkTargetConnected(link) {
+    if (!this.#referrerUrl || !this.#referrerLabel) { return }
+
+    const allowedPaths = (link.dataset.turboNavigationAllowedReferrerPaths || "").split(",")
+    const referrerPath = new URL(this.#referrerUrl).pathname
+    if (!allowedPaths.includes(referrerPath)) { return }
+
+    link.href = this.#referrerUrl
+    const strong = link.querySelector("strong")
+    if (strong) { strong.textContent = `Back to ${this.#referrerLabel}` }
+  }
+
   get #referrerPath() {
     if (!this.#referrerUrl) return null
     return new URL(this.#referrerUrl).pathname
@@ -24,4 +41,8 @@ export default class extends Controller {
   get #referrerUrl() {
     return sessionStorage.getItem("referrerUrl")
   }
+
+  get #referrerLabel() {
+    return sessionStorage.getItem("referrerLabel")
+  }
 }