feat(branches): add force delete flow for local branches

Branch deletion now supports an optional force mode in the Tauri
command, enabling deletion of branches that are not fully merged.
The UI replaces the simple confirm prompt with a dedicated dialog and
auto-escalates to force delete when Git rejects a normal delete.

- Add force flag to delete_branch command and tests
- Implement BranchDeleteConfirmDialog and wire it into App.svelte
- Update branch context menu actions and styling

src/lib/git.ts

@@ -72,8 +72,8 @@ export function renameBranch(
   return invoke<GitStatus>("rename_branch", { path, oldBranch, newBranch });
 }
 
-export function deleteBranch(path: string, branch: string): Promise<GitStatus> {
-  return invoke<GitStatus>("delete_branch", { path, branch });
+export function deleteBranch(path: string, branch: string, force = false): Promise<GitStatus> {
+  return invoke<GitStatus>("delete_branch", { path, branch, force });
 }
 
 export function stageFiles(path: string, files: string[]): Promise<GitStatus> {