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>

website/src/pages/index.astro

@@ -141,7 +141,7 @@ import Base from '../layouts/Base.astro';
           </div>
           <h3 class="text-base font-semibold">Live Visibility</h3>
           <p class="mt-2 text-sm leading-relaxed text-zinc-400">
-            Watch each agent think, delegate to sub-agents, invoke skills, and run hooks in real time. A context-usage bar shows how much of the model's window you've used.
+            Watch each agent think, delegate to sub-agents, invoke skills, and run hooks in real time. Track premium request consumption, per-agent token usage, and account quota remaining alongside the context-window bar.
           </p>
         </div>