refactor(events): change connectionUpdated payload from Void to UUID? for per-connection filtering

[datlechin]

Summary

AppEvents.connectionUpdated payload changes from PassthroughSubject<Void, Never> to PassthroughSubject<UUID?, Never>. The payload is the affected connection's id, or nil for bulk updates where the sender doesn't track individual ids.

Senders updated (9 sites):

• Single-connection: ConnectionFormCoordinator.deleteCurrent, ConnectionFormCoordinator.save (insert + update branches), sample-DB launcher (open + reset), per-connection iCloud-sync toggle in WelcomeContextMenus -> pass connection.id.
• Bulk: SyncCoordinator.applyRemoteChanges, ConnectionExportService.importConnections, multi-select iCloud-sync toggle in WelcomeContextMenus -> pass nil.

Subscribers: WelcomeViewModel is the only current subscriber and it discards the payload ({ _ in self?.loadConnections() }); the welcome list shows every connection so a payload-agnostic refresh is correct. The new payload type is binary-source-compatible with that closure shape.

Why this matters

Audit finding X2. The audit recommends matching the same shape as ConnectionStatusChange and DatabaseDidConnect, which already carry the affected connectionId so per-connection subscribers can filter. Today there are no per-connection subscribers, but the same anti-pattern that PR #1139 had to fix (mainCoordinatorTeardown that lacked a payload) and PR #1148 had to fix (databaseDidConnect subscribers that ignored the payload) starts here: every event the welcome list has to consume is also visible to any future per-coordinator subscriber, and without a payload that subscriber would have to refresh on every unrelated edit.

Test plan

• Edit a connection name on the welcome window; confirm the welcome list refreshes.
• Run a full sync pull that updates many connections; confirm welcome list refreshes once via nil event.
• Open the sample database; confirm welcome refreshes.
• Bulk toggle iCloud-sync on multiple selected connections via context menu; confirm welcome refreshes.
• swiftlint --strict clean.