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/contracts/ipc.ts

@@ -1,5 +1,5 @@
 import type { ApprovalDecision } from '@shared/domain/approval';
-import type { SidecarCapabilities, InteractionMode, MessageMode } from '@shared/contracts/sidecar';
+import type { SidecarCapabilities, InteractionMode, MessageMode, QuotaSnapshot } from '@shared/contracts/sidecar';
 import type { PatternDefinition, ReasoningEffort } from '@shared/domain/pattern';
 import type { ProjectRecord } from '@shared/domain/project';
 import type { QuerySessionsInput, SessionQueryResult } from '@shared/domain/sessionLibrary';
@@ -202,6 +202,7 @@ export interface ElectronApi {
   resizeTerminal(input: ResizeTerminalInput): void;
   openAppDataFolder(): Promise<void>;
   resetLocalWorkspace(): Promise<WorkspaceState>;
+  getQuota(): Promise<Record<string, QuotaSnapshot>>;
   onTerminalData(listener: (data: string) => void): () => void;
   onTerminalExit(listener: (info: TerminalExitInfo) => void): () => void;
   onWorkspaceUpdated(listener: (workspace: WorkspaceState) => void): () => void;