feat(submodules): add submodule management and recursive clone

Add a new backend module to manage Git submodules (list, initialize/update,
stage, sync, add) and expose Tauri commands (list_submodules,
submodule_action, add_submodule). The implementation enforces safe relative
paths, a maximum nesting depth, and guards (dirty/conflicted checks and
initialization/no-op semantics) to avoid unsafe operations.

Refactor clone logic to a testable run_git_clone_command and enable
automatic initialization of submodules during clone by passing
--recurse-submodules. Also disallow certain submodule-related custom clone
flags so callers cannot override this behaviour.

Update README with a Submodules section and add frontend components and types
to surface submodule UI (dialogs, toolbar badge). Unit tests were added for
submodule discovery, initialization/update semantics, and recursive clone
behavior.

README.md

@@ -22,10 +22,29 @@ Fast, simple, and designed for developers who want a clean Git experience withou
 - 📦 Repository management
 - ☁️ GitHub, GitLab, Azure DevOps, and Gitea integrations
 - 🗄️ Git LFS detection, tracking and object management
+- 🧩 Submodule management, including nested repositories
 - 🎨 Modern and intuitive UI
 
 ---
 
+## Submodules
+
+Cloning automatically downloads and initializes submodules, including nested
+submodules, at their recorded commits. The Submodules toolbar badge counts modules
+that still need initialization. After a successful pull, Gitty offers to initialize
+missing modules; choosing **Later** keeps the badge visible.
+
+Open a repository and select **Submodules** in the repository toolbar. The dialog
+shows each submodule's recorded commit (from its parent's index), checked-out
+commit, and local changes. You can add a submodule, initialize it, check out its
+recorded commit, stage a changed reference, synchronize its URL from `.gitmodules`,
+or open it as a repository tab. Nested submodules are included by default.
+
+Checking out a recorded commit is blocked when the submodule has local changes;
+commit or stash them in that repository first. Adding a submodule stages
+`.gitmodules` and the new reference. Commit these changes in the parent repository.
+Network operations use your configured Git credential helpers or SSH credentials.
+
 ## 📸 Preview
 
 > Screenshots comes later.