feat(frontend): add step cutoff control to export options

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

frontend/src/pages/SessionDetailPage.tsx

@@ -31,6 +31,7 @@ export function SessionDetailPage() {
   const [isSavingRatings, setIsSavingRatings] = useState(false)
   const [librarySteps, setLibrarySteps] = useState<Step[]>([])
   const [copiedStepIndex, setCopiedStepIndex] = useState<number | null>(null)
+  const [maxStepIndex, setMaxStepIndex] = useState<number | null>(null)
 
   useEffect(() => {
     if (id) {
@@ -94,6 +95,7 @@ export function SessionDetailPage() {
       format: exportFormat,
       include_timestamps: true,
       include_tree_info: true,
+      ...(maxStepIndex !== null && { max_step_index: maxStepIndex }),
     }
     return await sessionsApi.export(session.id, options)
   }
@@ -139,6 +141,7 @@ export function SessionDetailPage() {
         format: 'psa',
         include_timestamps: true,
         include_tree_info: true,
+        ...(maxStepIndex !== null && { max_step_index: maxStepIndex }),
       }
       const content = await sessionsApi.export(session.id, options)
       if (content) {
@@ -385,6 +388,24 @@ export function SessionDetailPage() {
                 <option value="html">HTML</option>
                 <option value="psa">PSA / Ticket Note</option>
               </select>
+              {session.decisions.length > 1 && (
+                <select
+                  value={maxStepIndex ?? ''}
+                  onChange={(e) => setMaxStepIndex(e.target.value ? Number(e.target.value) : null)}
+                  aria-label="Export through step"
+                  className={cn(
+                    'rounded-md border border-white/10 bg-black/50 px-3 py-2 text-sm text-white',
+                    'focus:border-white/30 focus:outline-none focus:ring-1 focus:ring-white/20'
+                  )}
+                >
+                  <option value="">All steps</option>
+                  {session.decisions.map((_, idx) => (
+                    <option key={idx + 1} value={idx + 1}>
+                      Through step {idx + 1}
+                    </option>
+                  ))}
+                </select>
+              )}
               <button
                 onClick={handleCopy}
                 disabled={isExporting}