refactor(web): redesign assistant management UI

[bobleer]

Summary

• align the assistant management surface with BitFun design tokens and existing product patterns
• clarify assistant configuration, new-session, and delete actions while adding loading, empty, error, and busy states
• reuse the shared destructive confirmation flow and update localized copy plus focused interaction tests
• treat the permission-mode control preference as optional so older configs no longer emit a backend ERROR

Validation

• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run src/app/scenes/profile/views/AssistantCard.test.tsx src/infrastructure/config/services/ConfigManager.test.ts
• pnpm run i18n:audit
• pnpm run theme:color-audit:all
• pnpm run desktop:dev

Visual QA

• verified dark and light themes
• verified the 720px responsive layout without horizontal overflow
• verified the redesigned assistant surface is served from the branch in desktop development mode
• verified the missing app.flow_chat.show_permission_mode_control preference no longer appears in desktop or webview error logs