feat: widget stack remembers its position across reboots

[jawzf]

Description

Follow-up to #1328 (widget stack).

Before: each stack's current widget was only kept in memory, so after a reboot, an app update or the process being killed, every stack went back to its first widget.

After: a stack comes back showing the widget it was on.

How: positions are saved per stack in the existing widget_stack_prefs file (position_<stackWidgetId>), next to the stack configs, and restored when StackHost starts. A small StackPositions.restore clamps each saved position to the stack's current widgets and drops positions for stacks that no longer exist. Positions are saved:

• right after an arrow or edge step,
• when the screen turns off,
• when the app is moved to the background (onTrimMemory), because a cached app gets frozen and doesn't receive SCREEN_OFF until it wakes.

They aren't saved on every timer tick, so a running stack doesn't write to disk every few seconds. Removing a stack also removes its saved position.

Type of change

• Bug fix
• New feature
• UI / visual update
• Performance improvement
• Refactoring / cleanup
• Translations
• Build / CI tooling

UI changes

None.

Checklist

• The base branch is set to develop (not main).
• The app builds cleanly locally with ./gradlew assembleDebug.
• Unit tests pass with ./gradlew testDebugUnitTest (new StackPositionsTest, 6 tests).
• Tested on the Android 17 emulator: stepped a 3-widget stack to the 2nd widget, rebooted, same widget shown; removed a widget so the saved position was out of range, rebooted, last widget shown without errors; confirmed nothing is written on timer ticks.
• Positions stay in WidgetStackRepository alongside the stack configs, as the widget stack already stores its data there; no new user-facing preference.
• No new settings, so nothing to register in FeatureRegistry.kt.
• No UI changes.
• No new strings or drawables.

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