refactor: independent sessions for page-split + URL-based pipeline navigation

Page-split now creates independent sessions (no parent_session_id),
parent marked as status='split' and hidden from list. Navigation uses
useSearchParams for URL-based step tracking (browser back/forward works).
page.tsx reduced from 684 to 443 lines via usePipelineNavigation hook.

Box sub-sessions (column detection) remain unchanged.

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

klausur-service/backend/ocr_pipeline_sessions.py

@@ -191,12 +191,12 @@ async def get_session_info(session_id: str):
     if session.get("ground_truth"):
         result["ground_truth"] = session["ground_truth"]
 
-    # Sub-session info
+    # Box sub-session info (zone_type='box' from column detection — NOT page-split)
     if session.get("parent_session_id"):
         result["parent_session_id"] = session["parent_session_id"]
         result["box_index"] = session.get("box_index")
     else:
-        # Check for sub-sessions
+        # Check for box sub-sessions (column detection creates these)
         subs = await get_sub_sessions(session_id)
         if subs:
             result["sub_sessions"] = [