feat(ui): shared header — Sign in/Sign out and New Chat on every page; hide the Sources nav link from anonymous users

tests/e2e/test_chat_persistence.py

@@ -271,12 +271,14 @@ def test_persists_across_page_navigation(
     _ask(page, QUESTION)
     _ask_deflected(page, OFF_TOPIC)  # mixed conversation: grounded + deflected
 
-    # A trip to Sources — the New chat control is chat-page-only.
-    # (Phase 16: the catalog is admin-only — the trip starts with a
-    # real form login.)
+    # A trip to Sources (phase 16: the catalog is admin-only — the trip
+    # starts with a real form login). Phase 19 (owner permission
+    # 2026-08-23): the New chat control is part of the SHARED bar, so it
+    # is present on the sources page too — only clicking it would clear
+    # the conversation (pinned in tests/e2e/test_shared_header.py).
     login(page, app_url, next="/sources.html")
     expect(page.locator("#docs-tbody tr").first).to_be_visible(timeout=15_000)
-    expect(page.locator("#new-chat-btn")).to_have_count(0)
+    expect(page.locator("#new-chat-btn")).to_be_visible()
 
     # Back to the chat: the conversation is exactly as left — both turns,
     # the source chip, and the amber deflected bubble with its chips.