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.

src/lib/types.ts

@@ -491,3 +491,18 @@ export interface IntegrationBoard {
 export interface IntegrationBoardReference { title: string; webUrl: string; scope: string }
 
 export interface IssueComment { id: string; author: string; body: string; createdAt: string; bodyHtml: boolean }
+
+export interface GitSubmodule {
+  name: string;
+  path: string;
+  owner_path: string;
+  relative_path: string;
+  full_path: string;
+  url: string;
+  branch: string | null;
+  recorded_commit: string;
+  local_commit: string | null;
+  dirty: boolean;
+  conflicted: boolean;
+  depth: number;
+}