refactor(branch-dropdown): drop worktree parent-branch merge shortcut

Remove the parent-branch button shown beside the branch selector along
with its supporting API, command, service, model, and i18n strings.
Add migration m20260423 to drop the now-unused folder.parent_branch
column.

src-tauri/src/db/entities/folder.rs

@@ -15,7 +15,6 @@ pub struct Model {
     pub updated_at: DateTimeUtc,
     pub deleted_at: Option<DateTimeUtc>,
     pub is_open: bool,
-    pub parent_branch: Option<String>,
     pub sort_order: i32,
 }