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

docs/architecture.md

@@ -1060,6 +1060,7 @@ Git dashboard routes are registered in `register-git-github.ts`.
 | POST | `/api/git/pull` | Pull the current branch and return structured conflict metadata on merge conflicts. |
 | POST | `/api/git/push` | Push the current branch. |
 | GET | `/api/git/stashes` | List stash entries. |
+| GET | `/api/git/stashes/:index/diff` | Return stash stat + patch for a validated stash index (404 when missing). |
 | POST | `/api/git/stashes` | Create a stash with an optional message. |
 | POST | `/api/git/stashes/:index/apply` | Apply a stash by index (optionally drop after apply via `drop: true`). |
 | DELETE | `/api/git/stashes/:index` | Drop a stash by index. |

docs/dashboard-guide.md

@@ -126,6 +126,7 @@ Features:
 - Commit and diff browsing
 - Push/pull/fetch actions
 - Remote editing controls
+- Stash inspection (view stat + patch) before apply/pop/drop actions
 - Remotes tab keeps "Recent commits on {remote}" in sync immediately after successful push/pull actions
 
 ![Git manager](./screenshots/git-manager.png)