refactor(database-switcher): rebuild as native macOS popover anchored to toolbar

[datlechin]

Summary

• Database switcher: replaced the modal .sheet with a native NSPopover-equivalent anchored to the ⌘K toolbar button. Selection-as-action (click row to switch), no Cancel/Open footer. Search field, Refresh, Drop, and New Database all available within the popover.
• Create Database sheet: removed NavigationStack and .formStyle(.grouped) (iOS-style with dark-mode background bug). Replaced with manual VStack + .formStyle(.columns) matching Mail "New Mailbox" / System Settings "Add Network" pattern. Width 380pt, hug-content height.
• Drop Database: deleted the custom DropDatabaseSheet. Replaced with native .confirmationDialog attached to the main view.

Backup/restore flows

DatabaseSwitcherSheet slimmed to backup/restore handoff only — wrapped in NavigationStack with .cancellationAction / .confirmationAction toolbar items, checkmark indicator instead of custom "current" pill.

Test plan

• Press ⌘K — popover opens anchored to the database toolbar button
• Click the chip in the toolbar center — same popover opens, anchored to ⌘K button
• Switch tab Databases ↔ Schemas — popover stays the same size (no reflow)
• Type in search field — list filters in real time, text size matches body font
• Press ↑ ↓ — list navigation works from search field
• Press Return — switches to selected database/schema and dismisses
• Press Esc with text — clears search; press Esc empty — dismisses popover
• Click "+ New Database…" — popover dismisses, Create Database sheet opens from main window
• Create Database sheet renders correctly in both light and dark mode (no white background bug)
• Right-click a non-current database row — "Drop Database…" context menu appears, confirmation dialog is native style
• PostgreSQL backup/restore flow still works (uses DatabaseSwitcherSheet)
• ⌘N inside popover opens Create Database; ⌘R refreshes the list