chore: bump version to v2026.4.309 and update changelog}
- Bump version from v2026.4.308 to v2026.4.309 in package.json
- Add v2026.4.309 changelog entry documenting OnPush change detection and CSP/toast updates
- Update build version references across README, artifacts, changelogs, and contributors docs
- Add worker-src 'self' blob: directive to Content Security Policy in Electron app
- Adjust toast notification positioning (16px -> 8px) and padding (10px/20px -> 8px/16px)