Summary

• Parse ACP UsageUpdate events and cache ACP session context usage separately from token usage reports
• Emit ACP context usage updates to the frontend and display usage percentage in the ACP model selector
• Add coverage for ACP context usage DTO/store handling and fix the UserMessageItem test mock for FlowChatStore

Verification

• cargo test -p bitfun-acp
• cargo check -p bitfun-desktop
• pnpm run lint:web
• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run