Replace inner Navigator with Overlay (fixes #31, #35) — v5.0.0

[tomalabaster]

Summary

Revives the overlay-based approach from feature/#31_-_use_overlay_instead_of_navigator, rebased onto current develop (4.3.0), as a 5.0.0 release.

AppLock no longer wraps the app in a nested Navigator. Lock and inactive screens are layered via OverlayEntry instead, leaving MaterialApp's navigator as the sole route stack.

Fixes

• Hero Widget Animation is not working After using AppLock (flutter_app_lock : 4.1.1+1) #31 — Hero animations work again (no nested navigator breaking overlay flights)
• After implementing AppLock, the device back button closes the app instead of returning to the previous screen (Android) #35 — Android back correctly pops routes after dismissing dialogs
• Option to customise or remove animation on Navigator push or pop. #20 (side effect) — Lock/unlock no longer uses navigator push/pop slide animations

Architecture

MaterialApp
└── AppLock
    └── Overlay
        ├── Entry: app (MaterialApp Navigator — sole route stack)
        ├── Entry: lock screen (when locked)
        └── Entry: inactive screen (when inactive)

Notable implementation details

• Imperative overlay sync — single _syncOverlays() path; avoids mixing initialEntries with manual insert/remove (root cause of iOS background-lock bugs on the old WIP branch)
• Deferred background lock — if the latency timer fires while the app is still backgrounded, lock is shown on next resumed (addresses iOS reports on After implementing AppLock, the device back button closes the app instead of returning to the previous screen (Android) #35)
• Offstage app content — when lock or inactive overlay is visible, app content stays mounted (maintainState) but is offstage, matching prior navigator semantics for tests and state preservation
• Breaking API cleanup — removed deprecated lockScreen, enabled, and backgroundLockLatency constructor parameters (slated for 5.0.0)

Test plan

• All 88 existing widget tests pass
• 6 new overlay/regression tests (test/src/app_lock_overlay_test.dart):
  • Overlay entry mount state
  • Deferred background lock on resume
  • InactiveBehavior.alwaysShow when disabled
  • Hero navigation through AppLock
  • Android back after dialog dismiss
• flutter analyze clean
• Manual verification on Android (dialog + back, Hero)
• Manual verification on iOS (background lock with latency)

Example app adds Hero test and Dialog then back test buttons for manual QA.

Migration (4.x → 5.0)

No integration pattern change — still use MaterialApp.builder. Replace any remaining deprecated constructor args:

Removed (4.x)	Use instead
lockScreen	lockScreenBuilder
enabled	initiallyEnabled
backgroundLockLatency	initialBackgroundLockLatency

Closes #31, #35