Make Hub synchronization state truthful

[sarthakagrawal927]

Closes #20.

What changed

• separates local iPhone storage, temporary iCloud device mirroring, and the private Significant Hobbies Hub
• exposes durable waiting-change count from PersonalSyncKit
• reports active sync, last successful sync, and actionable authentication/network/service failures
• stops treating sign-in as proof that data synchronized
• preserves local writes when iCloud or Hub updates fail
• adds focused model and Connection-screen coverage

Verification

• KithCore tests: 9 passed
• KithAppTests: passed
• focused Connection UI journey: passed
• unsigned Release simulator build: passed
• git diff --check: passed

The full keyboard-driven UI suite is left to GitHub Actions. On this machine, both Xcode 26.6/iOS 26.5 and Xcode 27 beta/iOS 27 block in UIKit pasteboard XPC when XCUITest focuses a text field; the non-keyboard Connection journey completes normally.