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)

desktop/src/lib/saas-client.ts

@@ -132,6 +132,7 @@ import { installAdminMethods } from './saas-admin';
 import { installRelayMethods } from './saas-relay';
 import { installPromptMethods } from './saas-prompt';
 import { installTelemetryMethods } from './saas-telemetry';
+import { installBillingMethods } from './saas-billing';
 
 // === Client Implementation ===
 
@@ -411,6 +412,7 @@ installAdminMethods(SaaSClient);
 installRelayMethods(SaaSClient);
 installPromptMethods(SaaSClient);
 installTelemetryMethods(SaaSClient);
+installBillingMethods(SaaSClient);
 
 // === API Method Type Declarations ===
 // These methods are installed at runtime by installXxxMethods() in saas-*.ts.
@@ -516,6 +518,10 @@ export interface SaaSClient {
       timestamp: string;
     }>;
   }): Promise<{ accepted: number; total: number }>;
+
+  // --- Billing (saas-billing.ts) ---
+  incrementUsageDimension(dimension: string, count?: number): Promise<import('./saas-billing').UsageIncrementResult>;
+  reportUsageFireAndForget(dimension: string, count?: number): void;
 }
 
 // === Singleton ===