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/models/folder.rs

@@ -17,7 +17,6 @@ pub struct FolderDetail {
     pub name: String,
     pub path: String,
     pub git_branch: Option<String>,
-    pub parent_branch: Option<String>,
     pub default_agent_type: Option<AgentType>,
     pub last_opened_at: DateTime<Utc>,
     pub sort_order: i32,