refactor: replace GCD dispatch patterns with Swift structured concurrency

[datlechin]

Summary

Replaces DispatchQueue patterns with Swift structured concurrency (Task, Task.sleep) across 16 files. No behavior changes.

Critical fix

• SQLDialectFactory.createDialect(for:) — removed @MainActor isolation (was only needed because it called through PluginManager.shared, now calls PluginMetadataRegistry.shared directly which is thread-safe)
• SQLFormatterService.resolveDialectProvider — eliminated DispatchQueue.main.sync deadlock risk and Thread.isMainThread check, now a simple direct call

GCD → Task replacements (11 sites)

• EditorEventRouter — next-run-loop deferral
• SQLEditorCoordinator — 4 sites: prepareCoordinator, inline suggestions, throttled frame change (DispatchWorkItem → Task + Task.sleep), selection scroll
• DataGridCoordinator — settings observer callback
• AppDelegate+WindowConfig — flattened nested DispatchQueue.main.async { Task {} } into single Task
• MainEditorContentView — 4 sites: window dirty indicator, provider cache ops, column layout
• MainContentView — sidebar sync

asyncAfter → Task.sleep (7 files)

HistoryPanelView, SQLPreviewSheet, SyncStatusIndicator, SQLReviewPopover, ConnectionExportOptionsSheet, WelcomeWindowView, AIChatCodeBlockView

Intentionally NOT changed

• SSH/TOTP semaphores (synchronous blocking required by SSH handshake)
• Storage init semaphores (synchronous init, not in async context)
• AppKit delegate callbacks (dispatching to main from non-@mainactor contexts)

Test plan

• Open editor, type SQL — verify autocomplete/inline suggestions work
• Resize editor — verify frame change notifications fire correctly
• Connect to database — verify auto-reconnect at startup works
• Change DataGrid settings — verify row height/font changes apply
• Edit column layout — verify layout saves persist
• History panel, SQL preview, sync indicator — verify timing-based UI still works