feat(submodules): support auth and checkout submodule revisions

Add credential-aware submodule operations and a command to checkout a specific
tag or commit in a submodule without staging the parent repository.

- Backend (src-tauri):
  - Export checkout_submodule_revision and implement checkout_revision which
    validates tag vs commit inputs, verifies refs locally, and checks out the
    submodule in detached mode without modifying the parent's index.
  - Add optional username/password parameters to add_submodule and submodule_action
    flows. Implement submodule_git to call run_git_authenticated when credentials
    are supplied and classify auth failures by prefixing errors with "AUTH_FAILED:".
  - Wire authenticated variants (operate_authenticated, add_authenticated) and
    update fetch/update actions to use credentials where needed.
  - Add unit tests covering authenticated submodule commands, auth failure
    classification, and checkout-by-tag/commit behavior.

- Frontend:
  - App.svelte: introduce credential prompt flow (withSubmoduleCredentials,
    submit/cancel handlers), surface credential dialog on auth failures, and
    wire credentialed calls for initialize/add/update/fetch operations. Hook up
    checkoutSubmoduleRevision and listTags to the submodule dialog.
  - SubmoduleDialog.svelte: add UI for selecting destination folder, loading
    tags and checking out revisions; expose fetch action.
  - CredentialDialog.svelte: include "submodule" action and adjust labels.

- Docs:
  - README: document "Change commit or tag" and "Fetch tags & commits" behaviors.

The commit focuses only on enabling credentialed submodule interactions and
safe local checkouts of tags/commits; no other git behavior changes are made.

src-tauri/src/main.rs

@@ -10,7 +10,7 @@ use badge::set_sync_badge;
 use external_tools::{
     detect_external_tools, launch_external_diff, launch_external_merge, launch_external_tool,
 };
-use git::submodules::{add_submodule, list_submodules, submodule_action};
+use git::submodules::{checkout_submodule_revision, add_submodule, list_submodules, submodule_action};
 use git::{
     SearchCancellationState, add_remote, add_to_gitignore, add_worktree, amend_commit,
     apply_file_patch, cancel_code_search, cancel_file_history, checkout_branch, cherry_pick_abort,
@@ -366,6 +366,7 @@ async fn main() {
             list_submodules,
             add_submodule,
             submodule_action,
+            checkout_submodule_revision,
             list_worktrees,
             add_worktree,
             remove_worktree,