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/m20260423_000001_drop_folder_parent_branch.rs

@@ -0,0 +1,35 @@
+use sea_orm_migration::prelude::*;
+
+#[derive(DeriveMigrationName)]
+pub struct Migration;
+
+#[async_trait::async_trait]
+impl MigrationTrait for Migration {
+    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .alter_table(
+                Table::alter()
+                    .table(Folder::Table)
+                    .drop_column(Folder::ParentBranch)
+                    .to_owned(),
+            )
+            .await
+    }
+
+    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .alter_table(
+                Table::alter()
+                    .table(Folder::Table)
+                    .add_column(ColumnDef::new(Folder::ParentBranch).string().null())
+                    .to_owned(),
+            )
+            .await
+    }
+}
+
+#[derive(DeriveIden)]
+enum Folder {
+    Table,
+    ParentBranch,
+}

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),
         ]
     }
 }