refactor(toolbar): make Active Connections a toolbar popover instead of a modal (#1350)

[datlechin]

Fixes #1350.

Problem

Opening Active Connections showed a modal that dimmed the background with no visible close button, so it was unclear how to dismiss it.

Root cause

The switcher was presented as a SwiftUI .sheet. A sheet is modal by design: it dims and blocks the parent and does not light-dismiss on click-outside, so it needs an explicit close control, which it lacked. The wrong primitive was the real problem, not a missing button.

Approach

Rewrite it as a native popover anchored to the toolbar's Connection button, mirroring the existing DatabaseSwitcher (which already does this). A SwiftUI .popover light-dismisses on Escape and click-outside automatically and resets its binding, so no close button is needed (this matches the macOS HIG: a popover is a transient, source-anchored view and only needs a close button for save/cancel tasks). The Cmd+Control+C menu path lights up the same popover with no extra work, exactly like Cmd+K does for the database switcher. TablePlus, the closest native analog, uses the same pattern: a toolbar-anchored, searchable, keyboard-navigable popover with light dismiss.

Changes

• MainContentCoordinator: add isConnectionSwitcherShown; remove .connectionSwitcher from the shared ActiveSheet.
• MainContentCommandActions: openConnectionSwitcher() sets the flag.
• MainContentView: drop the .connectionSwitcher sheet case.
• MainWindowToolbar+Buttons: ConnectionToolbarButton carries the .popover, bound to coordinator state.
• ConnectionSwitcherSheet.swift becomes ConnectionSwitcherPopover.swift: drops the dead isPresented binding and .onExitCommand; adds a search field, arrow-key navigation, and Return to switch; keeps the active/saved sections, tap to activate, and Manage Connections. Ctrl+J/K is replaced by the arrow keys.
• Pure ConnectionSwitcherFilter.matches(_:query:) extracted for the search.

QuickSwitcherSheet stays a sheet (it is a full-content command palette, correctly modal).

Tests

ConnectionSwitcherFilterTests: empty/whitespace query matches all, case-insensitive substring name match, database-name match, and non-matching query returns false.

Docs

CHANGELOG under Changed. docs/databases/overview.mdx already described the switcher as a popover; updated it with the search, keyboard, and Escape/click-outside dismissal details so the docs match the implementation.