What does this PR do?

Add a 1×1 home screen check-in widget. Users can bind a check-in mode habit to the widget and tap the icon to quickly check in / uncheck today without opening the app.

Also fixes two data-sync gaps between widgets and the app:

• Widget → App: GroveModel now calls SharedPreferences.reload() and re-reads data when the app resumes, so changes made by the widget are picked up immediately.
• App → Widget: widget_bridge.dart now always calls updateWidgets, even if rendering a single habit tree fails.

Type of change

• Bug fix
• New feature

How has this been tested?

Tested on a real Android device with the following scenarios:

1. Added a check-in habit → added the widget from launcher → habit picker shows "Check-In Widget" label.
2. Selected a check-in habit → widget shows habit name, circle icon, and streak count.
3. Tapped the icon → toggled ✓ (colored) / ○ (grey). Streak count updated correctly.
4. Opened the app → habit card reflected the changes made from the widget.
5. Checked in from the app → widget updated after updatePeriodMillis / force refresh.
6. Tapped widget background (not icon) → no app launch.
7. Selected an abstain-mode habit → widget showed "—" and was non-interactive.

Checklist

• Tested on a real Android device
• CHANGELOG.md updated under [Unreleased]
• No new dependencies that phone home or collect data
• Code follows the existing style