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/hooks/use-export-table.tsx

@@ -10,6 +10,34 @@ import useAPIKeysColumns from './use-apikeys-columns';
 import useModelsColumns from './use-models-columns';
 import useUsersColumns from './use-users-columns';
 
+type ColumnLike = {
+  title: any;
+  dataIndex: string | string[];
+  key?: string;
+};
+
+const toFieldKey = (dataIndex: string | string[]): string =>
+  Array.isArray(dataIndex) ? dataIndex.join('_') : dataIndex;
+
+const buildSheetMeta = (columns: ColumnLike[]) => {
+  const fields: string[] = [];
+  const fieldLabels: Record<string, any> = {};
+  const formatMap: Record<string, (raw: any, row: any) => any> = {};
+
+  columns.forEach((col) => {
+    if (!col.dataIndex) return;
+    const key = toFieldKey(col.dataIndex);
+    fields.push(key);
+    fieldLabels[key] = col.title;
+    if (Array.isArray(col.dataIndex)) {
+      const path = col.dataIndex;
+      formatMap[key] = (_raw, row) => _.get(row, path);
+    }
+  });
+
+  return { fields, fieldLabels, formatMap };
+};
+
 const useExportTable = () => {
   const store = useStore();
   const modelsColumns = useModelsColumns();
@@ -20,53 +48,34 @@ const useExportTable = () => {
     const usersTableData = store.get(usersTableDataAtom);
     const apiKeysTableData = store.get(apiKeysTableDataAtom);
     const modelsTableData = store.get(modelsTableDataAtom);
+
+    const modelsMeta = buildSheetMeta(modelsColumns);
+    const apiKeysMeta = buildSheetMeta(apiKeysColumns);
+    const usersMeta = buildSheetMeta(usersColumns);
+
     exportJsonToExcel({
       fileName: 'table_data.xlsx',
       sheets: [
         {
           jsonData: modelsTableData.dataList || [],
           sheetName: 'models',
-          fields: modelsColumns
-            .map((col: any) => _.join(col.dataIndex, '_'))
-            .filter(Boolean) as string[],
-          fieldLabels: modelsColumns.reduce(
-            (map, col) => {
-              map[_.join(col.dataIndex, '_')] = col.title;
-              return map;
-            },
-            {} as Record<string, any>
-          ),
-          formatMap: {}
+          fields: modelsMeta.fields,
+          fieldLabels: modelsMeta.fieldLabels,
+          formatMap: modelsMeta.formatMap
         },
         {
           jsonData: apiKeysTableData.dataList || [],
           sheetName: 'api_keys',
-          fields: apiKeysColumns
-            .map((col: any) => col.dataIndex)
-            .filter(Boolean) as string[],
-          fieldLabels: apiKeysColumns.reduce(
-            (map, col) => {
-              map[_.join(col.dataIndex, '_')] = col.title;
-              return map;
-            },
-            {} as Record<string, any>
-          ),
-          formatMap: {}
+          fields: apiKeysMeta.fields,
+          fieldLabels: apiKeysMeta.fieldLabels,
+          formatMap: apiKeysMeta.formatMap
         },
         {
           jsonData: usersTableData.dataList || [],
           sheetName: 'users',
-          fields: usersColumns
-            .map((col: any) => col.dataIndex)
-            .filter(Boolean) as string[],
-          fieldLabels: usersColumns.reduce(
-            (map, col) => {
-              map[_.join(col.dataIndex, '_')] = col.title;
-              return map;
-            },
-            {} as Record<string, any>
-          ),
-          formatMap: {}
+          fields: usersMeta.fields,
+          fieldLabels: usersMeta.fieldLabels,
+          formatMap: usersMeta.formatMap
         }
       ]
     });