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/pages/SharedSessionPage.tsx

@@ -97,7 +97,7 @@ export function SharedSessionPage() {
           const status = err.response?.status
           if (status === 401) {
             navigate('/login', {
-              state: { from: `/share/${shareToken}` },
+              state: { from: { pathname: `/share/${shareToken}` } },
               replace: true,
             })
             return
@@ -232,7 +232,7 @@ export function SharedSessionPage() {
           <div className="lg:col-span-2">
             <SessionTimeline
               decisions={data.decisions}
-              treeType={data.tree_structure?.tree_type as string}
+              treeType={data.tree_structure?.tree_type}
               startedAt={data.started_at}
               completedAt={data.completed_at}
               showCopyButtons={false}