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>

admin-lehrer/components/ocr-pipeline/BoxSessionTabs.tsx

@@ -21,6 +21,7 @@ function getStatusIcon(sub: SubSession): string {
   return STATUS_ICONS.pending
 }
 
+/** Tabs for box sub-sessions (from column detection zone_type='box'). */
 export function BoxSessionTabs({ parentSessionId, subSessions, activeSessionId, onSessionChange }: BoxSessionTabsProps) {
   if (subSessions.length === 0) return null
 
@@ -28,7 +29,6 @@ export function BoxSessionTabs({ parentSessionId, subSessions, activeSessionId,
 
   return (
     <div className="flex items-center gap-1.5 px-1 py-1.5 bg-gray-50 dark:bg-gray-800/50 rounded-xl border border-gray-200 dark:border-gray-700">
-      {/* Main session tab */}
       <button
         onClick={() => onSessionChange(parentSessionId)}
         className={`px-3 py-1.5 rounded-lg text-xs font-medium transition-colors ${
@@ -42,7 +42,6 @@ export function BoxSessionTabs({ parentSessionId, subSessions, activeSessionId,
 
       <div className="w-px h-5 bg-gray-200 dark:bg-gray-700" />
 
-      {/* Sub-session tabs */}
       {subSessions.map((sub) => {
         const isActive = activeSessionId === sub.id
         const icon = getStatusIcon(sub)
@@ -59,7 +58,7 @@ export function BoxSessionTabs({ parentSessionId, subSessions, activeSessionId,
             title={sub.name}
           >
             <span className="mr-1">{icon}</span>
-            Seite {sub.box_index + 1}
+            Box {sub.box_index + 1}
           </button>
         )
       })}