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/preload/index.ts

@@ -65,6 +65,7 @@ const api: ElectronApi = {
   selectSession: (sessionId) => ipcRenderer.invoke(ipcChannels.selectSession, sessionId),
   openAppDataFolder: () => ipcRenderer.invoke(ipcChannels.openAppDataFolder),
   resetLocalWorkspace: () => ipcRenderer.invoke(ipcChannels.resetLocalWorkspace),
+  getQuota: () => ipcRenderer.invoke(ipcChannels.getQuota),
   onTerminalData: (listener) => {
     const handler = (_event: Electron.IpcRendererEvent, data: Parameters<typeof listener>[0]) =>
       listener(data);