refactor: replace timing hacks with structured signals

[datlechin]

Summary

Replaces 6 timing hacks (polling loops, arbitrary delays) with proper structured alternatives:

Site	Before	After
configureWelcomeWindow()	5x50ms polling for welcome window	Removed — windowDidBecomeKey already handles styling
Phase 2 queries	200ms Task.sleep before row count/enum fetch	Task(priority: .background) — proper deprioritization
Tab switch	16ms Task.sleep for onChange settle	Task.yield() — yields without fixed duration
Connection form focus	10x20ms polling NSApp.windows	NSWindow.didBecomeKeyNotification observer
Share file import	300ms asyncAfter before notification	Pending URL pattern — setUp() consumes stored URL
WindowOpener readiness	25x200ms polling for openWindow != nil	CheckedContinuation with 5s timeout via TaskGroup

Test plan

• Launch app → welcome window styled correctly (no polling needed)
• Run a query → row count appears in status bar (background priority)
• Switch between tabs → smooth, no delay
• Click New Connection from welcome → connection form window focuses
• Open .tablepro file → import sheet appears on welcome window
• Deep link with connection URL → connects after window system ready
• Launch with queued URLs → all processed correctly