feat(ui): documents open in an almost-fullscreen modal instead of a new page — same-page overlay on chat + Sources, /document.html kept as the no-JS/direct-link fallback

tests/e2e/test_chat_persistence.py

@@ -158,7 +158,7 @@ def test_conversation_survives_reload(
     chip = page.locator(".msg.brain .source-chip", has_text="kubernetes.md")
     expect(chip).to_have_count(1)
     expect(chip.first).to_have_attribute("href", CHIP_HREF)
-    expect(chip.first).to_have_attribute("target", "_blank")
+    expect(chip.first).not_to_have_attribute("target")  # phase 26: modal, not a new tab
 
     # The restore is read-only: storage still holds the same two messages.
     assert [m["who"] for m in _stored_parsed(page)["messages"]] == ["user", "brain"]