fix: persist branch viewing index in store to survive phase remounts

Local useState resets to 0 every time phase transitions from 'generating'
back to 'detailing', causing the view to snap back to branch 1.

Move viewingIndex to store's currentBranchIndex (already existed) and
advance it in generateBranchDetail after success. Component reads from
store so remounts no longer lose position.

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

frontend/src/store/aiFlowBuilderStore.ts

@@ -136,8 +136,12 @@ export const useAIFlowBuilderStore = create<AIFlowBuilderState>()((set, get) =>
       const updatedBranches = selectedBranches.map((b) =>
         b.name === branchName ? { ...b, steps: response.steps } : b
       )
+      // Advance to the next branch that still needs detail
+      const nextIndex = updatedBranches.findIndex((b) => !b.steps)
+      const currentBranchIndex = nextIndex !== -1 ? nextIndex : updatedBranches.findIndex((b) => b.name === branchName)
       set({
         selectedBranches: updatedBranches,
+        currentBranchIndex,
         phase: 'detailing',
         isLoading: false,
       })