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/components/daily-usage.tsx

@@ -104,7 +104,7 @@ const DailyUsage: React.FC<DailyUsageProps> = (props) => {
       };
     }
 
-    const groupDim = groupBy as 'user' | 'model' | 'api_key' | null;
+    const groupDim = groupBy as 'user' | 'route' | 'api_key' | null;
     const isCached = metric === CACHED_METRIC;
 
     const dateSet = new Set<string>(