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/components/CloneRepositoryDialog.svelte

@@ -346,7 +346,7 @@
                   {:else}
                     <label class="clone-option-detail"><span>{isGerman ? "Seit" : "Since"}</span><input type="date" bind:value={shallowSince} disabled={isBusy} aria-invalid={!shallowSince.trim()} /></label>
                   {/if}
-                  <label class="clone-option-detail"><span>{isGerman ? "Zusätzliche Flags" : "Custom flags"}</span><input bind:value={customFlags} autocomplete="off" spellcheck="false" placeholder="--recurse-submodules --single-branch" disabled={isBusy} /></label>
+                  <label class="clone-option-detail"><span>{isGerman ? "Zusätzliche Flags" : "Custom flags"}</span><input bind:value={customFlags} autocomplete="off" spellcheck="false" placeholder="--single-branch" disabled={isBusy} /></label>
                   <small class="clone-option-help">{isGerman ? "Flags wie in der Git-Kommandozeile; verwaltete oder unsichere Flags werden abgewiesen." : "Enter flags as on the Git command line; managed or unsafe flags are rejected."}</small>
                 </div>
               {/if}