ui: add test-id for add-empty-commit and wire in menu

Added TestId for the branch header context menu 'Add empty commit' action and wired the testId into the BranchHeaderContextMenu Svelte component.

Changes:
- packages/ui/src/lib/utils/testIds.ts: added BranchHeaderContextMenu_AddEmptyCommit constant.
- apps/desktop/src/components/BranchHeaderContextMenu.svelte: set testId={TestId.BranchHeaderContextMenu_AddEmptyCommit} on the 'Add empty commit' ContextMenuItem.

These changes are UI/test scaffolding only and enable e2e tests to interact with the add-empty-commit action.

Author: estib-vega

apps/desktop/src/components/BranchHeaderContextMenu.svelte

@@ -237,6 +237,7 @@
 				<ContextMenuItem
 					label="Add empty commit"
 					icon="new-empty-commit"
+					testId={TestId.BranchHeaderContextMenu_AddEmptyCommit}
 					onclick={async () => {
 						await insertBlankCommitInBranch({
 							projectId,

packages/ui/src/lib/utils/testIds.ts

@@ -24,6 +24,7 @@ export enum TestId {
 	BranchHeaderAddDependanttBranchModal = 'branch-header-add-dependent-branch-modal',
 	BranchHeaderAddDependanttBranchModal_ActionButton = 'branch-header-add-dependent-branch-modal-action-button',
 	BranchHeaderContextMenu_SquashAllCommits = 'branch-header-context-menu-squash-all-commits',
+	BranchHeaderContextMenu_AddEmptyCommit = 'branch-header-context-menu-add-empty-commit',
 	EditCommitMessageBox = 'edit-commit-message-box',
 	CommitDrawer = 'commit-drawer',
 	CommitDrawerActionUncommit = 'commit-drawer-action-uncommit',