fix: semantic selected-text colors and proper full-screen CommandGroup

[datlechin]

Summary

Two quick-win fixes from the codebase audit:

• Semantic selected-text color — Replaced 15 hardcoded .white foreground colors in selected/highlighted row states with Color(nsColor: .alternateSelectedControlTextColor). Fixes broken appearance in "Increase Contrast" accessibility mode and with custom accent colors. Affected views: QuickSwitcherView, ConnectionSwitcherPopover, DatabaseSwitcherSheet.

• Full-screen CommandGroup — Replaced NSEvent.addLocalMonitorForEvents hack for Ctrl+Cmd+F with a proper CommandGroup(after: .toolbar) button. The menu system now owns the shortcut, enabling proper menu validation and the standard "Enter Full Screen" / "Exit Full Screen" title toggle.

Test plan

• Quick Switcher (Cmd+P): navigate with arrows, selected row text readable in light/dark mode
• Connection Switcher popover: highlighted row appearance correct
• Database Switcher (Cmd+K): selected row appearance correct
• Enable "Increase Contrast" in Accessibility settings → selected rows still readable
• Ctrl+Cmd+F toggles full-screen correctly
• View menu shows "Enter Full Screen" with ⌃⌘F shortcut
• In full-screen, View menu shows "Exit Full Screen"