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/migration/mod.rs

@@ -11,6 +11,7 @@ mod m20260404_000001_model_provider;
 mod m20260406_000001_agent_setting_model_provider;
 mod m20260420_000001_opened_tabs;
 mod m20260422_000001_folder_sort_order;
+mod m20260423_000001_drop_folder_parent_branch;
 pub struct Migrator;
 
 #[async_trait::async_trait]
@@ -28,6 +29,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260406_000001_agent_setting_model_provider::Migration),
             Box::new(m20260420_000001_opened_tabs::Migration),
             Box::new(m20260422_000001_folder_sort_order::Migration),
+            Box::new(m20260423_000001_drop_folder_parent_branch::Migration),
         ]
     }
 }