feat(FN-3623): add stash diff viewing UI and backend contract

Merges FN-3623 stash diff inspection system (backend contract, GitManagerModal viewing UI with new API route and tests) and FN-3088 dependency graph hover/selection highlighting. The stash diff work dominates the commit count and touches the core GitManagerModal, while the graph plugin gains new hig

Fusion-Task-Id: FN-3623

packages/dashboard/app/api/legacy.ts

@@ -2398,6 +2398,11 @@ export function dropStash(index: number, projectId?: string): Promise<{ message:
   });
 }
 
+/** Fetch stash diff (stat + patch) */
+export function fetchStashDiff(index: number, projectId?: string): Promise<{ stat: string; patch: string }> {
+  return api<{ stat: string; patch: string }>(withProjectId(`/git/stashes/${index}/diff`, projectId));
+}
+
 /** Fetch unstaged diff (working directory changes) */
 export function fetchUnstagedDiff(projectId?: string): Promise<{ stat: string; patch: string }> {
   return api<{ stat: string; patch: string }>(withProjectId("/git/diff", projectId));