feat: separate system task token usage and add usage refresh

Add a System tasks tab to run history that splits background usage (supervisor, auto-supervision, magic notes, knowledge base, delegation) from conversation usage, with a manual refresh button and automatic refresh when usage is recorded. Record previously dropped usage from supervisor reviews, image regeneration, knowledge graph extraction, and remote embeddings.

src/shared/contracts.ts

@@ -2159,6 +2159,8 @@ export type DesktopApi = {
   }
   usage: {
     getTokenSummary: () => Promise<TokenUsageSummary>
+    /** Fires (debounced) after any model usage is recorded. */
+    onChanged: (listener: () => void) => () => void
   }
   artifacts: {
     list: (projectId?: string) => Promise<AssistantArtifact[]>