feat(billing): add usage increment API + wire hand/pipeline execution tracking

Server side:
- POST /api/v1/billing/usage/increment endpoint with dimension whitelist
  (hand_executions, pipeline_runs, relay_requests) and count validation (1-100)
- Returns updated usage quota after increment

Desktop side:
- New saas-billing.ts mixin with incrementUsageDimension() and
  reportUsageFireAndForget() (non-blocking, safe for finally blocks)
- handStore.triggerHand: reports hand_executions after successful run
- PipelinesPanel.handleRunComplete: reports pipeline_runs on completion
- SaaSClient type declarations for new billing methods

Billing pipeline now covers all three dimensions:
  relay_requests  → relay handler (server-side, real-time)
  hand_executions → handStore (client-side, fire-and-forget)
  pipeline_runs   → PipelinesPanel (client-side, fire-and-forget)

crates/zclaw-saas/src/billing/mod.rs

@@ -13,6 +13,7 @@ pub fn routes() -> axum::Router<crate::state::AppState> {
         .route("/api/v1/billing/plans/{id}", get(handlers::get_plan))
         .route("/api/v1/billing/subscription", get(handlers::get_subscription))
         .route("/api/v1/billing/usage", get(handlers::get_usage))
+        .route("/api/v1/billing/usage/increment", post(handlers::increment_usage_dimension))
         .route("/api/v1/billing/payments", post(handlers::create_payment))
         .route("/api/v1/billing/payments/{id}", get(handlers::get_payment_status))
         // 支付回调（无需 auth）