perf(launch): defer non-critical services to first activation

[datlechin]

Summary

• Welcome window memory footprint at launch: 60.7 MB → 47.9 MB (-12.8 MB, -21%), peak 65.6 → 54.0 MB
• Source: WWDC 2019 Optimizing App Launch — "Defer anything unrelated to generating the first frame". Apple's Mac App Programming Guide says the same: "Any objects that are not needed at launch time should be loaded later."
• This PR moves non-critical services out of applicationDidFinishLaunching to either the first applicationDidBecomeActive (one-shot) or the view's first .onAppear, with idempotency guards on each service so re-entry is a no-op

Changes

• AppDelegate.swift — adds hasRunPostLaunchActivation guard, splits services across two phases
  • Stays in applicationDidFinishLaunching: theme, sync-settings keychain flag, DatabaseManager.startObservingSystemEvents, MemoryPressureAdvisor, PluginManager.loadPlugins (lazy bundle.load() is a separate PR), MCP server (already conditional), QueryHistoryStorage warm (already detached), launch coordinator, notification observers
  • Deferred to first applicationDidBecomeActive: ConnectionStorage.migratePluginSecureFieldsIfNeeded, AnalyticsService.startPeriodicHeartbeat, SyncCoordinator.start, LinkedFolderWatcher.start, LicenseManager.startPeriodicValidation
• FavoritesTabView.swift — SQLFolderWatcher.start() moved to .onAppear of the Favorites tab; users who never open Favorites never spin up the FSEvents stream
• SyncCoordinator.swift — hasStarted guard so start() is idempotent even if called repeatedly
• LinkedFolderWatcher.swift / SQLFolderWatcher.swift — hasStarted guard cleared on stop() so reload() still works correctly

Test plan

• Cold launch: Welcome appears, Settings does not (regression check vs fix(windows): prevent Settings from auto-opening on launch #1022)
• After ~1s, sync runs (Pull starting, Sync completed successfully in logs)
• Open Favorites sidebar tab: SQL folder watcher starts, linked SQL files appear
• Add/remove a linked SQL folder: SQLFolderWatcher.shared.reload() still works (stop clears hasStarted, start re-runs)
• License validation runs in background (no UI block)
• swiftlint --strict on changed files: 0 violations
• xcodebuild Release arm64: BUILD SUCCEEDED
• vmmap before/after: Physical footprint 60.7 MB → 47.9 MB

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