fix(ui): document viewer back button returns to the page you came from (chat or sources)

tests/e2e/test_document_viewer.py

@@ -99,10 +99,11 @@ def test_source_chip_opens_document(
 
     chip = page.locator(".msg.brain .source-chip", has_text="kubernetes.md")
     expect(chip).to_have_count(1, timeout=30_000)
-    # New-tab contract: same-origin viewer URL, both query values encoded
-    # (the path's slashes come out as %2F — exactly why encoding matters).
+    # New-tab contract: same-origin viewer URL, all query values encoded
+    # (the path's slashes come out as %2F — exactly why encoding matters),
+    # plus back=/ (phase 13) so the viewer's back button returns to chat.
     expect(chip.first).to_have_attribute(
-        "href", "/document.html?source=docs&path=homelab%2Fkubernetes.md"
+        "href", "/document.html?source=docs&path=homelab%2Fkubernetes.md&back=%2F"
     )
     expect(chip.first).to_have_attribute("target", "_blank")
     expect(chip.first).to_have_attribute("rel", "noopener")
@@ -112,7 +113,9 @@ def test_source_chip_opens_document(
     viewer = popup_info.value
     expect(viewer).to_have_url(
         re.compile(
-            re.escape(f"{app_url}/document.html?source=docs&path=homelab%2Fkubernetes.md")
+            re.escape(
+                f"{app_url}/document.html?source=docs&path=homelab%2Fkubernetes.md&back=%2F"
+            )
         )
     )
     expect(viewer.locator("#doc-title")).to_have_text("Kubernetes Homelab Cluster")