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/shared/domain/event.ts

@@ -1,5 +1,7 @@
 import type { SessionRunRecord } from '@shared/domain/runTimeline';
 
+import type { QuotaSnapshot } from '@shared/contracts/sidecar';
+
 export type SessionActivityType = 'thinking' | 'tool-calling' | 'handoff' | 'completed';
 
 export type SessionEventKind =
@@ -14,7 +16,8 @@ export type SessionEventKind =
   | 'hook-lifecycle'
   | 'session-usage'
   | 'session-compaction'
-  | 'pending-messages-modified';
+  | 'pending-messages-modified'
+  | 'assistant-usage';
 
 export type SubagentEventKind = 'started' | 'completed' | 'failed' | 'selected' | 'deselected';
 
@@ -67,4 +70,15 @@ export interface SessionEventRecord {
   preCompactionTokens?: number;
   postCompactionTokens?: number;
   tokensRemoved?: number;
+
+  // Assistant usage fields
+  usageModel?: string;
+  usageInputTokens?: number;
+  usageOutputTokens?: number;
+  usageCacheReadTokens?: number;
+  usageCacheWriteTokens?: number;
+  usageCost?: number;
+  usageDuration?: number;
+  usageTotalNanoAiu?: number;
+  usageQuotaSnapshots?: Record<string, QuotaSnapshot>;
 }