refactor: replace hacks with native macOS patterns and clean up concurrency

[datlechin]

Summary

• Toolbar: Replace .opacity() / .allowsHitTesting() hacks with conditional if rendering for database switcher, import button, and results button. Merge connection + database into a single toolbar item group so non-switching databases (libSQL, SQLite, Redis) show no gap. Hide "Switch Database" hint in empty state for non-switching databases.
• SSH dialogs: Replace DispatchSemaphore blocking pattern with DispatchQueue.main.sync in PromptPassphraseProvider and PromptTOTPProvider — same semantics, no thread waste.
• Terminal: Replace @State NSObjectProtocol? observer with .task using NotificationCenter.default.notifications(named:) async stream for SSH tunnel waiting — auto-cancelled on view disappear, no manual lifecycle.
• Concurrency: Remove redundant Task { @MainActor in } across 28 files where the enclosing type is already @MainActor. Keep @MainActor annotation in @Sendable closure contexts (NotificationCenter callbacks, KVO observers, DispatchSource handlers, Timer blocks).
• Timers: Replace DispatchQueue.main.asyncAfter with Task.sleep in MCPSection (copy confirmation) and FavoritesSidebarViewModel (folder rename delay).

Test plan

• Connect to MySQL/PostgreSQL — verify connection, queries, data editing
• Connect to libSQL/SQLite/Redis — verify no database switcher gap in toolbar
• Connect via SSH with encrypted key — passphrase dialog appears and works
• Connect via SSH with TOTP — verification code dialog appears and works
• Cancel SSH dialogs — fails gracefully, no hang
• Open terminal tab on SSH connection — waits for tunnel, then connects
• Close terminal tab while waiting — no crash or leak
• Export table data to CSV/JSON/SQL
• Import a SQL file
• Favorites sidebar — create folder, rename, add favorite
• Quick Switcher (Cmd+P) — search and select
• Settings > MCP section — copy button shows checkmark then resets
• Settings > Plugins — install/uninstall plugin
• Welcome window — open, create connection
• About window — open About TablePro
• ER diagram — open and view
• Switch between query and table tabs — results button visibility
• Multiple windows with different connections