Fix: Settings title stuck when backing out of sub-menus

[d4rken]

What changed

Fixed a bug on Android 16 where the Settings header stayed on a sub-page's name (e.g. "CorpseFinder") after backing out to the main Settings list with the system back button. The page content returned correctly — only the title stayed stuck. The toolbar's own back arrow was not affected.

Technical Context

• Root cause: on Android 16's predictive-back path, the new fragment reaches RESUMED before childFragmentManager.backStackEntryCount has been decremented. Both onBackStackChanged and onFragmentResumed see a stale count.
• The old code kept a parallel screens: ArrayList<Screen> mirroring the back stack and updated the toolbar from addOnBackStackChangedListener. On Android 16 the listener fired once with the pre-pop count and never fired again after the actual pop, so the toolbar stayed on the sub-page's title.
• New approach: register a FragmentLifecycleCallbacks and, on each child onFragmentResumed, read the title directly from the fragment's own arguments (BKEY_SCREEN_TITLE). The root fragment has no such arg, so it falls back to the default "Settings". No back-stack-count reads in the hot path — no more stale-value race.
• Scope is limited to SettingsFragment.kt. Shared infra (PreferenceFragment2, ToolbarHost) is untouched — SqueezerSettingsFragment uses PreferenceFragment2 as a top-level nav route without a ToolbarHost parent, and this fix shouldn't leak into that path.
• Verified on an Android 16 emulator: forward nav, system back, toolbar up arrow, and the Settings → Support → Contact form → back path (regression guard from #60dd3e66a) all land on the correct title/subtitle.

Closes #2386