feat: add per-app usage filtering (Claude/Codex/Gemini)

Add dashboard-level app type filter to usage statistics, replacing the
DataSourceBar with a more useful segmented control. All components
(summary cards, trend chart, provider stats, model stats, request logs)
now respond to the selected app filter.

Backend: add optional app_type parameter to get_usage_summary,
get_daily_trends, get_provider_stats, and get_model_stats queries.
Frontend: new AppTypeFilter type, updated query keys with appType
dimension for proper cache separation, and RequestLogTable local
filter auto-locks when dashboard filter is active.

src/components/usage/ModelStatsTable.tsx

@@ -11,12 +11,16 @@ import { useModelStats } from "@/lib/query/usage";
 import { fmtUsd } from "./format";
 
 interface ModelStatsTableProps {
+  appType?: string;
   refreshIntervalMs: number;
 }
 
-export function ModelStatsTable({ refreshIntervalMs }: ModelStatsTableProps) {
+export function ModelStatsTable({
+  appType,
+  refreshIntervalMs,
+}: ModelStatsTableProps) {
   const { t } = useTranslation();
-  const { data: stats, isLoading } = useModelStats({
+  const { data: stats, isLoading } = useModelStats(appType, {
     refetchInterval: refreshIntervalMs > 0 ? refreshIntervalMs : false,
   });