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/RepoToolbar.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import {
     Box,
+    Boxes,
     Bug,
     ChevronDown,
     Code2,
@@ -44,6 +45,8 @@
   export let onFetchPrune: () => void = () => {};
   export let onForcePush: () => void = () => {};
   export let onSyncOptions: () => void = () => {};
+  export let uninitializedSubmoduleCount = 0;
+  export let onOpenSubmodules: () => void = () => {};
   export let onOpenLfs: () => void = () => {};
 
   let historyOpen = false;
@@ -51,6 +54,9 @@
   let toolbarElement: HTMLDivElement;
 
   $: isGerman = language === "de";
+  $: submoduleLabel = uninitializedSubmoduleCount > 0
+    ? (isGerman ? `Submodule verwalten – ${uninitializedSubmoduleCount} nicht initialisiert` : `Manage submodules – ${uninitializedSubmoduleCount} not initialized`)
+    : (isGerman ? "Submodule verwalten" : "Manage submodules");
   $: pushLabel = localOnly ? (isGerman ? "Veröffentlichen" : "Publish") : "Push";
   $: pushTitle = localOnly
     ? (isGerman
@@ -220,6 +226,18 @@
         </div>
       {/if}
     </div>
+    <button
+      class="repo-action"
+      type="button"
+      onclick={onOpenSubmodules}
+      disabled={!hasRepository || isBusy}
+      title={submoduleLabel}
+      aria-label={submoduleLabel}
+    >
+      <Boxes size={15} aria-hidden="true" />
+      <span class="repo-action-label">{isGerman ? "Submodule" : "Submodules"}</span>
+      {#if uninitializedSubmoduleCount > 0}<span class="repo-action-count ahead" aria-hidden="true">{uninitializedSubmoduleCount}</span>{/if}
+    </button>
   </div>
 
   <div class="repo-toolbar-spacer"></div>