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/config/index.ts

@@ -6,7 +6,7 @@ export const groupByOptions = [
   //   label: 'None'
   // },
   {
-    value: 'model',
+    value: 'route',
     label: 'usage.filter.group.model'
   },
   {

src/pages/usage/config/types.ts

@@ -9,11 +9,13 @@ export interface UsageFilterItem {
       api_key_is_custom: boolean | null;
       provider_type: string | null;
       provider_name: string | null;
+      route_name: string | null;
     };
     current: {
       model_id: string | null;
       user_id: number | null;
       api_key_id: string | null;
+      route_id: number | null;
     };
   };
   label: string;
@@ -63,6 +65,7 @@ export type BreakdownItem = {
   provider_name: string;
   user: UsageFilterItem;
   model: UsageFilterItem;
+  route: UsageFilterItem;
   api_key: UsageFilterItem;
   date: {
     value: string;
@@ -88,6 +91,7 @@ export interface UsageMeta {
     models: UsageFilterItem[];
     users: UsageFilterItem[];
     api_keys: UsageFilterItem[];
+    routes: UsageFilterItem[];
   };
 }