feat(teams): add multi-select checkboxes and bulk delete to task list

Add checkbox column to task list view for selecting terminal-status
tasks (completed/failed/cancelled). Header checkbox supports select-all
with indeterminate state. Bulk action bar appears on selection with
delete button that opens ConfirmDeleteDialog requiring user to type
"delete" to confirm.

Backend: new teams.tasks.delete-bulk RPC method with DeleteTasks batch
SQL (DELETE ... WHERE id = ANY($1) RETURNING id). Broadcasts delete
event per task for real-time UI sync.

i18n: added bulk action keys for en/vi/zh.

pkg/protocol/methods.go

@@ -114,8 +114,9 @@ const (
 	MethodTeamsTaskComments  = "teams.tasks.comments"
 	MethodTeamsTaskEvents    = "teams.tasks.events"
 	MethodTeamsTaskCreate    = "teams.tasks.create"
-	MethodTeamsTaskDelete    = "teams.tasks.delete"
-	MethodTeamsTaskAssign    = "teams.tasks.assign"
+	MethodTeamsTaskDelete     = "teams.tasks.delete"
+	MethodTeamsTaskDeleteBulk = "teams.tasks.delete-bulk"
+	MethodTeamsTaskAssign     = "teams.tasks.assign"
 	MethodTeamsMembersAdd    = "teams.members.add"
 	MethodTeamsMembersRemove = "teams.members.remove"
 	MethodTeamsUpdate        = "teams.update"