feat: surface Copilot usage and quota data across the UI

Add three layers of usage visibility:

- ChatPane footer: premium request count, AIU consumed, and quota
  remaining below the existing context-window bar
- Settings / CopilotStatusCard: on-demand account quota section with
  progress bars, overage indicators, and reset dates fetched via
  the new get-quota sidecar command
- Activity Panel: per-agent token/cost/duration totals on each agent
  row and a Session Usage summary section between agents and timeline

Backend (sidecar):
- New get-quota command using SDK account.getQuota RPC
- New assistant-usage turn-scoped event from SDK assistant.usage
- QuotaSnapshotMapper for both typed and untyped SDK quota payloads
- DTOs: GetQuotaCommandDto, QuotaSnapshotDto, AccountQuotaResultEventDto,
  AssistantUsageEventDto

Frontend:
- Shared types: AssistantUsageEvent, QuotaSnapshot, GetQuotaCommand
- IPC bridge: getQuota channel, assistant-usage event dispatch
- State: SessionRequestUsageMap accumulator with per-agent breakdown
- 8 new tests for accumulator logic and formatting helpers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/AryxAppService.ts

@@ -1297,6 +1297,10 @@ export class AryxAppService extends EventEmitter<AppServiceEvents> {
     return queryWorkspaceSessions(workspace, input);
   }
 
+  async getQuota(): Promise<Record<string, import('@shared/contracts/sidecar').QuotaSnapshot>> {
+    return this.sidecar.getQuota();
+  }
+
   async refreshProjectGitContext(projectId?: string): Promise<WorkspaceState> {
     const workspace = await this.loadWorkspace();
     const projects = projectId
@@ -1893,6 +1897,24 @@ export class AryxAppService extends EventEmitter<AppServiceEvents> {
           agentName: event.agentName,
         });
         return;
+      case 'assistant-usage':
+        this.emitSessionEvent({
+          sessionId,
+          kind: 'assistant-usage',
+          occurredAt,
+          agentId: event.agentId,
+          agentName: event.agentName,
+          usageModel: event.model,
+          usageInputTokens: event.inputTokens,
+          usageOutputTokens: event.outputTokens,
+          usageCacheReadTokens: event.cacheReadTokens,
+          usageCacheWriteTokens: event.cacheWriteTokens,
+          usageCost: event.cost,
+          usageDuration: event.duration,
+          usageTotalNanoAiu: event.totalNanoAiu,
+          usageQuotaSnapshots: event.quotaSnapshots,
+        });
+        return;
     }
   }