Problem

Deleting a branch blocks the main thread because delete_branch runs synchronous git worktree removal on the Tauri main thread. This causes the UI to freeze until the operation completes.

Solution

Backend

• Make delete_branch a Tauri async command so the git worktree removal runs on a background thread instead of blocking the main thread.

Frontend

• Add a deleting state that shows a spinner overlay on the branch card while the backend operation runs.
• The confirm dialog dismisses immediately after the user confirms.
• The branch card shows a "Deleting…" spinner overlay while the async operation is in progress.
• On success, the card is removed from the list.
• On failure, the deleting state is cleared and the card returns to normal.

Changes

• src-tauri/src/lib.rs — Mark delete_branch as async
• src/lib/BranchCard.svelte — Accept deleting prop, render spinner overlay when active
• src/lib/ProjectHome.svelte — Track deletingBranches set, manage async delete lifecycle
• src/lib/ProjectSection.svelte — Pass deletingBranches through to BranchCard