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/types/usage.ts

@@ -123,6 +123,8 @@ export interface ProviderLimitStatus {
 
 export type TimeRange = "1d" | "7d" | "30d";
 
+export type AppTypeFilter = "all" | "claude" | "codex" | "gemini";
+
 export interface StatsFilters {
   timeRange: TimeRange;
   providerId?: string;