Configuration: Implement config restore functionality

[malpern]

Problem

Backup exists but restoration doesn't work - throws "Restore not yet implemented" error.

Location

ConfigurationManager.swift line 342

func restoreLastGoodConfig() async throws {
    throw KeyPathError.configuration(.loadFailed(
        reason: "Restore not yet implemented"))
}

Impact

Users can't recover from bad configurations using built-in restore.

Fix

1. Wire up backup restoration logic
2. Add "Restore Last Good Config" button in Settings
3. Test restoration flow

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md section 6.1