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/SidecarProtocolHost.cs

@@ -18,6 +18,7 @@ public sealed class SidecarProtocolHost
     private const string ListSessionsCommandType = "list-sessions";
     private const string DeleteSessionCommandType = "delete-session";
     private const string DisconnectSessionCommandType = "disconnect-session";
+    private const string GetQuotaCommandType = "get-quota";
     private const string AskUserToolName = "ask_user";
     private static readonly HashSet<string> ExcludedRuntimeToolNames = new(StringComparer.OrdinalIgnoreCase)
     {
@@ -82,6 +83,7 @@ public sealed class SidecarProtocolHost
             [ListSessionsCommandType] = HandleListSessionsAsync,
             [DeleteSessionCommandType] = HandleDeleteSessionAsync,
             [DisconnectSessionCommandType] = HandleDisconnectSessionAsync,
+            [GetQuotaCommandType] = HandleGetQuotaAsync,
         };
     }
 
@@ -313,6 +315,23 @@ public sealed class SidecarProtocolHost
         }, context.CancellationToken).ConfigureAwait(false);
     }
 
+    private async Task HandleGetQuotaAsync(CommandContext context)
+    {
+        _ = DeserializeCommand<GetQuotaCommandDto>(context);
+        IReadOnlyDictionary<string, QuotaSnapshotDto> quotaSnapshots =
+            await _sessionManager.GetQuotaAsync(context.CancellationToken).ConfigureAwait(false);
+
+        await WriteAsync(context.Output, new AccountQuotaResultEventDto
+        {
+            Type = "quota-result",
+            RequestId = context.Envelope.RequestId,
+            QuotaSnapshots = quotaSnapshots.ToDictionary(
+                snapshot => snapshot.Key,
+                snapshot => snapshot.Value,
+                StringComparer.Ordinal),
+        }, context.CancellationToken).ConfigureAwait(false);
+    }
+
     private TCommand DeserializeCommand<TCommand>(CommandContext context)
         where TCommand : SidecarCommandEnvelope
     {