Lets enterprise plugins contribute per-route columns and a global
mount point on the Usage page's Models tab. Mirrors the existing
modelRoutes pattern: `usage.modelsExtraColumns` for cells,
`<PluginExtraFields name="UsageModelsPageGlobal" />` for the
page-level data lifecycle (receives the visible route ids so a
plugin can bulk-fetch per-row data in one call).
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.
