refactor(usage): switch breakdown dimension from model to route

Align with gpustack backend commits 7b57cd3b (add route dimension to
usage breakdown) and 51fd25c8 (use model_route_id as models_called):

* group_by, filters, and BreakdownItem now use the `route` dimension;
  `models` filter cascader is replaced with a flat `routes` selector
  (routes have no provider/cluster grouping).
* Tab / group_by option / column header keep the user-facing "模型 / Model"
  wording — only the underlying data dimension changed.
* Drop cluster_name / provider_type columns from the breakdown table;
  drop api_keys_used column per design.
* Fix pre-existing excel export bug in use-export-table — string
  dataIndex was lodash-joined char-by-char ("input_tokens" →
  "i_n_p_u_t..."), array dataIndex was used as a flat key against nested
  rows. Rebuilt around buildSheetMeta that emits a stable field key plus
  a formatMap using _.get(row, path) for nested values. Sheet name kept
  as `models` to match the visible column label.

src/pages/usage/index.tsx

@@ -164,8 +164,8 @@ const Usage: React.FC = () => {
           filterBar.onDateChange(dates, dateStrings);
           handleBreakdownPageReset();
         }}
-        onModelsChange={(value) => {
-          filterBar.onModelsChange(value);
+        onRoutesChange={(value) => {
+          filterBar.onRoutesChange(value);
           handleBreakdownPageReset();
         }}
         onUsersChange={(value) => {
@@ -228,7 +228,7 @@ const Usage: React.FC = () => {
         handlePickerChange={handlePickerChange}
         onCancel={() => setOpenExportModal(false)}
         initialState={{
-          activeModels: filterBar.activeModels,
+          activeRoutes: filterBar.selectedRoutes,
           activeApiKeys: filterBar.activeApiKeys,
           users: commonFilters.users,
           start_date: commonFilters.start_date,