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/web/handlers/folders.rs

@@ -287,13 +287,6 @@ pub async fn open_push_window(
     }))
 }
 
-#[derive(Deserialize)]
-#[serde(rename_all = "camelCase")]
-pub struct SetFolderParentBranchParams {
-    pub path: String,
-    pub parent_branch: Option<String>,
-}
-
 pub async fn add_folder_to_history(
     Extension(state): Extension<Arc<AppState>>,
     Json(params): Json<AddFolderParams>,
@@ -305,16 +298,6 @@ pub async fn add_folder_to_history(
     Ok(Json(result))
 }
 
-pub async fn set_folder_parent_branch(
-    Extension(state): Extension<Arc<AppState>>,
-    Json(params): Json<SetFolderParentBranchParams>,
-) -> Result<Json<()>, AppCommandError> {
-    let db = &state.db;
-    folder_commands::set_folder_parent_branch_core(&db.conn, &params.path, params.parent_branch)
-        .await?;
-    Ok(Json(()))
-}
-
 pub async fn remove_folder_from_history(
     Extension(state): Extension<Arc<AppState>>,
     Json(params): Json<AddFolderParams>,

src-tauri/src/web/router.rs

@@ -114,10 +114,6 @@ pub fn build_router(state: Arc<AppState>, token: String, static_dir: std::path::
             "/add_folder_to_history",
             post(handlers::folders::add_folder_to_history),
         )
-        .route(
-            "/set_folder_parent_branch",
-            post(handlers::folders::set_folder_parent_branch),
-        )
         .route(
             "/remove_folder_from_history",
             post(handlers::folders::remove_folder_from_history),