feat(sync): CloudKit sync status indicator + crisper card hover

[pseudobun]

Summary

• Adds a small CloudKit sync status indicator to the TossesView toolbar (principal on macOS, top-leading on iOS). Shows a spinner while .import/.export events are in flight and an exclamationmark.icloud icon with a tooltip on error, auto-clearing on the next successful event. .setup events are filtered so cold launches don't spin.
• Implemented via a new @MainActor CloudSyncMonitor in TossKit that subscribes to NSPersistentCloudKitContainer.eventChangedNotification — the Apple-documented path for observing sync in a SwiftData + CloudKit stack (SwiftData's ModelContainer is still backed by NSPersistentCloudKitContainer and posts this notification globally).
• The indicator uses a fixed 18pt frame so the toolbar never reflows between idle and syncing, and opts out of the macOS/iOS 26 shared Liquid Glass background via .sharedBackgroundVisibility(.hidden) (gated with #available for older OSes).
• Replaces the 0.5% scaleEffect hover feedback on TossCard with a stronger shadow, an inner accent-colored border overlay, and a pointing-hand cursor. The fractional scale was forcing subpixel resampling and making the card look faintly blurry on hover.

Why

CloudKit sync is currently invisible to the user — when it silently broke recently (iOS↔macOS CloudKit environment mismatch) there was no signal at all. Apple's own guidance is that event notifications describe activity, not definitive "fully synced" truth, so the UX is deliberately minimal: transient spinner only, auto-clearing error icon, nothing persistent, no "last synced at X" text.

Test plan

• Launch on macOS; toolbar looks unchanged at idle (no bezel, no empty slot).
• Add a toss on the Mac; spinner appears briefly in the toolbar between "Tosses" and +, then disappears.
• Add a toss on a second device on the same iCloud account; first Mac shows a spinner when the .import event arrives and the toss appears in the list.
• Sign out of iCloud, add a toss; orange exclamationmark.icloud shows with tooltip. Sign back in and add another toss; icon auto-clears.
• Cold-launch several times; no spinner during .setup (filtered).
• Hover a toss card on macOS; border + shadow + cursor change, no blur.
• Build on iOS — .topBarLeading placement looks correct.

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