Refresh 0.4.0 release documentation

[dbuskariol]

Summary

• advance the immutable 0.4.0 candidate identity to build 15
• replace the pre-Time-Machine Permissions screenshot with the current six-row native surface
• clarify that tracked screenshots are product references, while exact-candidate screenshot proof belongs to generated manual acceptance evidence

Verification

• Scripts/verify-ci.sh
• 545 Swift tests passed with 3 documented opt-in skips
• real local restic lifecycle passed
• real rclone-backed Time Machine object lifecycle passed
• warning-free certificate-free Release app, strict nested signatures, FSKit contract, and release-policy self-tests passed

The exact installed build-14 UI was inspected before this documentation-only successor. External development-state investigation and generated release evidence remain outside the repository.