fix: address code review issues in session sharing

- Add useCallback for loadShares in ShareSessionModal (React hook deps)
- Use TreeStructure type instead of Record<string, unknown> for type safety
- Fix login redirect format to match LoginPage's expected state shape

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

frontend/src/components/session/SharedSessionTreePreview.tsx

@@ -1,7 +1,8 @@
+import type { TreeStructure } from '@/types'
 import { cn } from '@/lib/utils'
 
 interface SharedSessionTreePreviewProps {
-  treeStructure: Record<string, unknown>
+  treeStructure: TreeStructure
   pathTaken: string[]
 }