chore: address low-priority cleanup items

[datlechin]

Summary

Batch 5 (final) of anti-pattern fixes. Addresses 20 low-priority cleanup items.

Changes

• Replace deprecated .cornerRadius() with .clipShape(RoundedRectangle()) — 12 occurrences, 8 files
• Remove dead nonisolated deinit from AppDelegate
• Replace DispatchQueue.main.async with Task in @MainActor SQLEditorCoordinator — 4 occurrences
• Add @MainActor to UserDefaults+RecentDatabases.trackDatabaseAccess
• Add Logger warning on invalid hex color fallback in Color+Hex
• Document AppLanguage.apply() side effect
• Use ThemeEngine typography in ConnectionSwitcherPopover — replace 4 hardcoded font sizes

Skipped (intentional)

• String-based Selector("showSettingsWindow:") — standard macOS pattern, no alternative
• hasInitialized in MainContentView — guards tab restoration beyond coordinator init
• Task { @MainActor in } in DatabaseSwitcherSheet.moveSelection — needed for SwiftUI warning

Test plan

• Build succeeds
• No visual regressions in filter rows, import dialog, quick switcher, connection switcher

Summary by CodeRabbit

• New Features

  • Added SQL filter preview display for query builders.
  • Improved theme import with async loading.

• Bug Fixes

  • Fixed SSH host key verification flow during tunnel setup.
  • Enhanced password handling for transient database connections.
  • Improved CloudKit sync conflict detection and auto-resolution.

• Improvements

  • Updated database connection readiness checks.
  • Refactored plugin verification to execute asynchronously.
  • Improved server change token persistence.
  • Enhanced window and tab management consistency.
  • Updated UI styling for better visual consistency.