refactor(events): drop four 1-to-1 PassthroughSubject buses for direct ownership

[datlechin]

Summary

Four publishers on AppCommands had a single sender and a single receiver where direct ownership was already available. They are removed:

• saveAsFavoriteRequested — MainContentCoordinator+Favorites sent it, MainEditorContentView received it inside the same coordinator graph. Replaced with an @Observable favoriteDialogQuery: FavoriteDialogQuery? on the coordinator. The view binds the sheet's item: to it via Binding(get:set:).
• focusConnectionFormWindowRequested — sent in WelcomeViewModel.focusConnectionFormWindow(), received in AppDelegate.handleFocusConnectionForm(). The lookup (NSApp.windows.first(where: AppLaunchCoordinator.isConnectionFormWindow) → makeKeyAndOrderFront) is now inlined directly in WelcomeViewModel.
• openSampleDatabaseRequested — sent from the File menu, received in AppDelegate which just called SampleDatabaseLauncher.open(). Now the menu calls SampleDatabaseLauncher.open() directly.
• resetSampleDatabaseRequested — same shape, now calls SampleDatabaseLauncher.reset() directly.

Side effects:

• AppDelegate.handleFocusConnectionForm and the commandCancellables set are removed (no remaining subscribers).
• AppCommands.swift shrinks by four publisher declarations.

exportConnections, importConnections, importConnectionsFromApp (also flagged by the audit) are left in place: the menu fires regardless of whether the welcome view is loaded, so the publisher is genuinely 1-to-N (1-or-0 receivers depending on welcome window state) and benefits from the bus's auto-no-op-when-no-subscriber semantics.

Why this matters

This is audit finding 4.1, partial. Direct ownership is the stronger pattern when the receiver is reachable: it removes hidden subscribers, makes the call graph traceable in Xcode (jump-to-definition lands on the implementation, not on the publisher), and keeps AppCommands reserved for genuine fan-out.

Test plan

• Save current query as favorite via Cmd+D / menu and confirm the dialog still appears.
• Save selection-as-favorite from the editor context menu.
• Open and reset the sample database via the File menu.
• Use a welcome action that focuses the existing connection form window.
• swiftlint --strict clean on changed files.