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>

sidecar/src/Aryx.AgentHost/Services/CopilotTurnExecutionState.cs

@@ -127,6 +127,10 @@ internal sealed class CopilotTurnExecutionState
                     _pendingEvents.Enqueue(CreateHookLifecycleEvent(agent, "end", hookEnd.Data));
                 }
                 break;
+            case AssistantUsageEvent assistantUsage:
+                ActiveAgent = agent;
+                _pendingEvents.Enqueue(CreateAssistantUsageEvent(agent, assistantUsage.Data));
+                break;
             case SessionUsageInfoEvent usageInfo:
                 ActiveAgent = agent;
                 _pendingEvents.Enqueue(CreateUsageEvent(agent, usageInfo.Data));
@@ -410,6 +414,29 @@ internal sealed class CopilotTurnExecutionState
         };
     }
 
+    private AssistantUsageEventDto CreateAssistantUsageEvent(
+        AgentIdentity agent,
+        AssistantUsageData? data)
+    {
+        return new AssistantUsageEventDto
+        {
+            Type = "assistant-usage",
+            RequestId = _command.RequestId,
+            SessionId = _command.SessionId,
+            AgentId = agent.AgentId,
+            AgentName = agent.AgentName,
+            Model = data?.Model ?? string.Empty,
+            InputTokens = data?.InputTokens,
+            OutputTokens = data?.OutputTokens,
+            CacheReadTokens = data?.CacheReadTokens,
+            CacheWriteTokens = data?.CacheWriteTokens,
+            Cost = data?.Cost,
+            Duration = data?.Duration,
+            TotalNanoAiu = data?.CopilotUsage?.TotalNanoAiu,
+            QuotaSnapshots = QuotaSnapshotMapper.MapOrNull(data?.QuotaSnapshots),
+        };
+    }
+
     private SessionUsageEventDto CreateUsageEvent(AgentIdentity agent, SessionUsageInfoData? data)
     {
         return new SessionUsageEventDto