feat(connections): keyboard focus and VoiceOver in the connection form and welcome screen (#1490)

[datlechin]

Part of #1490 (keyboard, focus, accessibility). Connection form / welcome surface.

Keyboard focus

• Connection form opens with the Name field focused (@FocusState + .defaultFocus on the Form), so you can type immediately; Tab follows the natural field order.
• Welcome search field: Return or the Down arrow now hands focus to the connection list (wired the existing onSubmit/onMoveDown callbacks to set focus = .connectionList).
• Focus restore: after the welcome sheet (import, new group, etc.) dismisses, focus returns to the connection list.

VoiceOver / Full Keyboard Access

• Color swatches expose selected state (.accessibilityAddTraits(.isSelected)).
• Host-list entry fields get an accessibility label instead of an empty string.
• Onboarding Continue / Get Started buttons get .keyboardShortcut(.defaultAction) so Return activates them.

Native SwiftUI focus APIs only (@FocusState, .defaultFocus, .focused), no custom focus handling.

Deferred (follow-ups)

The shared NativeSearchField @FocusState refactor and the AIRulesEditor Tab-forwarding are higher-risk shared/representable changes; left for separate PRs.

Lint clean, style gate clean. Focus/responder behavior verified manually (not unit-testable here).