refactor: rebuild Welcome as Xcode-style launcher with HIG-correct primitives

[datlechin]

Why this exists

The Welcome window's runtime behavior is launcher-shaped: open the window, pick a connection, the connection opens in its own window, the launcher is dismissed/backgrounded. Apple's documented analogue is Xcode's Welcome window. PR #1052 picked the Mail/Notes-style NavigationSplitView + toolbar-search frame, which fixed the white-search-on-vibrancy bug correctly using documented APIs but forced a "sidebar isn't really a sidebar" mismatch (Apple HIG: sidebar = navigation; the Welcome left column is branding + a primary CTA).

This PR reverses the structural choice while keeping the API hygiene from #1052 and folds in audit findings (drop DoubleClickDetector, drop per-row .contextMenu, single sheet path for license activation, search debounce, single filter source, localization gaps, comments).

Architecture

• HStack { WelcomeActionsPanel, Divider, ConnectionsPanel }. No NavigationSplitView. No structural sidebar.
• Window: 820x500, .windowResizability(.contentSize), .windowStyle(.hiddenTitleBar). Launcher exception per HIG Launching page.
• Actions panel (left, 240pt, .regularMaterial): app icon + version + license + three action buttons (Create Connection prominent, Try Sample, Import Connections) + sync indicator + keyboard hints. Localized version line and Check-for-Updates link.
• Connections panel (right, opaque controlBackgroundColor): inline header with + / "New Group" buttons + NativeSearchField + connection list. Search field renders correctly because the panel background is opaque, not vibrancy.

HIG-correct primitives

Concern	API
Single-purpose launcher window	Window scene + .windowResizability(.contentSize)
Search field	NSSearchField via NativeSearchField wrapper, on opaque background
Cmd-F focus	.onKeyPress(characters: .init(charactersIn: "f")) on connections panel sets focus = .search
Double-click + context menu	contextMenu(forSelectionType: UUID.self, menu:, primaryAction:) on the List, identical pattern to FavoritesTabView.swift:145
Multi-select context menu	Selection set is passed to the menu closure, which dispatches to single/multi/empty branches
License activation	One sheet path through vm.activeSheet = .activation; SyncStatusIndicator accepts a callback
Search debounce	150ms Task cancellation; immediate rebuild on entering/leaving search
Hover affordance for "Activate License"	.buttonStyle(.link) (system link cursor) instead of custom NSCursor.push/pop

Audit cleanups

• Drop DoubleClickDetector from welcome surfaces (WelcomeConnectionRow.swift, linked-folder row).
• Drop per-row .contextMenu in TreeRowsView. Replaced by container-level contextMenu(forSelectionType:).
• Drop vm.filteredConnections (dual filter source); empty-state checks vm.treeItems.isEmpty && vm.linkedConnections.isEmpty.
• Drop searchText.didSet { rebuildTree() } synchronous path; replaced with debounced scheduleRebuildTree.
• Drop SyncStatusIndicator's local .sheet. Routes through callback.
• Drop three commented blocks: SyncStatusIndicator.swift:5-6 header, WelcomeConnectionRow.swift:108-113 doc, WelcomeViewModel.swift:156-157 planning note.
• Wrap unlocalized strings: Version %@, Check for Updates..., Pro, Activate License, New, Settings.
• Replace License expired — sync paused with License expired, sync paused (em-dash removed per CLAUDE.md style).
• Extract hasGroups computed in TreeRowsView to replace inline if case .group = $0 predicate.

Out of scope

• Other DoubleClickDetector callers (SidebarView.swift:196, DatabaseSwitcherSheet.swift:254, DatabaseTypeChooserSheet.swift:78, QuickSwitcherView.swift:171). Each warrants its own selection-type-driven refactor in a follow-up.
• "Recents" section. DatabaseConnection has no lastUsedAt; data-model change first.
• WindowChromeConfigurator / WindowOpenerBridge NSViewRepresentable bridges. SwiftUI has no first-class API for hiding individual standard window buttons; these are unavoidable.

CHANGELOG

No entry. The Welcome SwiftUI scene rework is already in [Unreleased] Changed; this PR refines that unreleased work, which CLAUDE.md says not to double-log. Same logic as #1052.

Source material

• Audit and HIG research notes saved in docs/refactor/welcome-hig-research.md and docs/refactor/welcome-hig-synthesis.md (already on a separate branch / not staged).
• Apple HIG: Sidebars, Search fields, Toolbars, Launching.
• Apple API: contextMenu(forSelectionType:menu:primaryAction:).

Test plan

• Welcome opens at 820x500, non-resizable, no title bar, no sidebar toggle button.
• Left (actions) panel shows three buttons; Create is prominent, Sample and Import are bordered.
• Right (connections) panel: + and "New Group" inline buttons, search field renders without white box bezel.
• Cmd-F focuses the search field. Esc clears.
• Type to filter; debounced (~150ms).
• Single-click selects, double-click connects (via primaryAction).
• Right-click on selected row in a multi-selection shows the multi-selection menu.
• Right-click on an unselected row shows the single-row menu (collapsed selection).
• Right-click on empty space in the list shows the new-connection menu.
• Empty state shows "Try Sample Database" CTA.
• License activation works from both the version-line link and the sync indicator.
• Linked folder rows still connect via the same primaryAction.

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