[codex] Add custom countdown items

[Evander764]

Summary

• add user-created countdown items with schema v2 migration and reducer support
• keep a single desktop countdown widget with pinned-item display and today-remaining fallback
• include local architecture handoff docs, storage safety, backup UI, windowing split, and patch-broadcast changes from the current local source state

Validation

• npm run typecheck
• npm test

Notes

This PR was prepared from a clean temporary clone because the local workspace has no commit history or origin remote and shows the whole checkout as untracked. Historical/generated folders such as backups, downloads, release, sources, coverage, out, and node_modules were not synced into the branch.