fix: help page back link preserves active persona

Pass ?persona= query param on the help link so the server knows which
persona to return to. Previously always defaulted to personas[0], causing
navigation back to the wrong persona.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/routers/help.py

@@ -32,13 +32,17 @@ def _get_session_user(request: Request) -> str | None:
 
 
 @router.get("/help", include_in_schema=False)
-async def help_page(request: Request):
+async def help_page(request: Request, persona: str = ""):
     username = _get_session_user(request)
     if not username:
         return RedirectResponse("/login", status_code=302)
 
     personas = list_user_personas(username)
-    back_persona = personas[0] if personas else ""
+    # Use persona from query param if valid, else fall back to first
+    if persona and persona in personas:
+        back_persona = persona
+    else:
+        back_persona = personas[0] if personas else ""
     back_href = f"/{username}/{back_persona}" if back_persona else "/"
 
     html = (_STATIC / "help.html").read_text()