fix: keep usage history snapshots enabled

[icebear0828]

Summary

• keep the usage stats UI on selected history-window totals
• add an independent usage_stats.snapshot_interval_minutes setting, defaulting to 5 minutes
• stop tying local usage history snapshots to quota.refresh_interval_minutes, so quota refresh can be disabled without making history stay at zero

Tests

• npx vitest run tests/unit/auth/usage-refresher.test.ts tests/unit/config-schema.test.ts tests/unit/config.test.ts
• cd web && npx vitest run src/pages/tests/usage-stats.test.tsx
• npm run build
• git diff --check