* fix(app): keep focused agent timelines live
Window focus was conflated with app visibility, so switching OS windows could remove the selected timeline subscription. Separate those signals, grace every visibility-driven removal, and cover retained history until authoritative catch-up completes.
* fix(app): keep timeline catch-up failures non-blocking
* fix(app): surface timeline catch-up failures
* fix(app): preserve file preview refocus
* fix(app): preserve optimistic catch-up flow
Add `packages/app/src/constants/platform.ts` with canonical gates
(isWeb, isNative, getIsElectron) and refactor all ~100 ad-hoc
Platform.OS checks across 69 files to use shared imports.
Fix sidebar hover crash on native (onPointerEnter → onHoverIn),
fix tooltip to use breakpoint instead of platform detection, make
sidebar action buttons always visible on native where hover doesn't
work, and document the platform gating decision matrix in CLAUDE.md.
