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/main/sidecar/sidecarProcess.ts

@@ -16,6 +16,7 @@ import type {
   RunTurnCommand,
   CopilotSessionListFilter,
   CopilotSessionInfo,
+  QuotaSnapshot,
 } from '@shared/contracts/sidecar';
 import type { ApprovalDecision } from '@shared/domain/approval';
 import type { ChatMessageRecord } from '@shared/domain/session';
@@ -80,6 +81,12 @@ type PendingCommand =
       resolve: () => void;
       reject: (error: Error) => void;
     })
+  | ({
+      processId: number;
+      kind: 'get-quota';
+      resolve: (snapshots: Record<string, QuotaSnapshot>) => void;
+      reject: (error: Error) => void;
+    })
   | ({
       processId: number;
     } & RunTurnPendingCommand);
@@ -185,6 +192,13 @@ export class SidecarClient {
     });
   }
 
+  async getQuota(): Promise<Record<string, QuotaSnapshot>> {
+    return this.dispatch<Record<string, QuotaSnapshot>>({
+      type: 'get-quota',
+      requestId: `get-quota-${Date.now()}`,
+    });
+  }
+
   async dispose(): Promise<void> {
     const state = this.processState;
     if (!state) {
@@ -341,6 +355,13 @@ export class SidecarClient {
           resolve: resolve as () => void,
           reject,
         });
+      } else if (command.type === 'get-quota') {
+        this.pending.set(command.requestId, {
+          processId: state.id,
+          kind: 'get-quota',
+          resolve: resolve as (snapshots: Record<string, QuotaSnapshot>) => void,
+          reject,
+        });
       } else {
         this.pending.set(command.requestId, {
           processId: state.id,
@@ -424,10 +445,17 @@ export class SidecarClient {
       case 'session-usage':
       case 'session-compaction':
       case 'pending-messages-modified':
+      case 'assistant-usage':
         if (pending.kind === 'run-turn' && shouldHandleRunTurnEvent(pending)) {
           this.invokeRunTurnHandler(event.requestId, pending, () => pending.onTurnScopedEvent(event));
         }
         return;
+      case 'quota-result':
+        if (pending.kind === 'get-quota') {
+          pending.resolve(event.quotaSnapshots);
+          this.pending.delete(event.requestId);
+        }
+        return;
       case 'sessions-listed':
         if (pending.kind === 'list-sessions') {
           pending.resolve(event.sessions);