fix: return pending_task_lane in session detail API response + debug logging

_build_session_detail was omitting pending_task_lane, is_branching, and
active_branch_id from the GET /ai-sessions/{id} response. The fields
existed on the schema and model but were never passed in the manual
constructor, so task lane state could never be restored on navigation.

Also adds console logging to AssistantChatPage selectChat flow to
diagnose message restoration, and fixes ScriptTemplateEditor stepper
dismiss firing during programmatic script_body updates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/script-editor/ScriptTemplateEditor.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react'
+import { useState, useEffect, useRef } from 'react'
 import { ArrowLeft, Loader2, Save, Scan, Trash2 } from 'lucide-react'
 import { Input } from '@/components/ui/Input'
 import { Textarea } from '@/components/ui/Textarea'
@@ -64,16 +64,19 @@ export function ScriptTemplateEditor({ templateId, onBack, onSaved }: Props) {
   const [detectedCandidates, setDetectedCandidates] = useState<ParameterCandidate[]>([])
   const [showStepper, setShowStepper] = useState(false)
   const [detectionSummary, setDetectionSummary] = useState<string | null>(null)
+  const acceptingCandidateRef = useRef(false)
 
   const { canShareScriptTemplate } = usePermissions()
 
-  // Dismiss stepper if user edits the script body during detection
+  // Dismiss stepper if user manually edits the script body during detection
+  // (but NOT when handleAcceptCandidate programmatically updates script_body)
   const scriptBodyRef = form.script_body
   useEffect(() => {
-    if (showStepper) {
+    if (showStepper && !acceptingCandidateRef.current) {
       setShowStepper(false)
       setDetectedCandidates([])
     }
+    acceptingCandidateRef.current = false
   // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [scriptBodyRef])
 
@@ -263,6 +266,7 @@ export function ScriptTemplateEditor({ templateId, onBack, onSaved }: Props) {
       sensitive: overrides.sensitive,
     }
 
+    acceptingCandidateRef.current = true
     setForm(f => ({
       ...f,
       script_body: updatedScript,