Add per-widget list selector with chevron dropdown

[pgarr]

Summary

• Widget header now shows a chevron icon (▾) next to the list title
• Tapping the chevron opens a dialog listing all available lists to choose from
• Selection is saved per widget instance in SharedPreferences (widget_prefs file, key selected_list_<widgetId>) — survives reboots
• Falls back to the most recently created list when no selection is saved or the saved list was deleted (stale pref auto-cleared)
• Prefs are cleaned up when a widget instance is removed (onDeleted)
• Multiple widget instances are fully independent

New files

• WidgetPrefs.kt — SharedPreferences helper
• WidgetListSelectActivity.kt — dialog Activity for the list picker
• widget_chevron_down.xml — Lucide-style chevron drawable

Modified files

• WidgetDbHelper.kt — added getAllLists(), getListById(), resolveList()
• NoteListWidget.kt — uses resolveList, wires chevron PendingIntent, handles onDeleted
• NoteListWidgetService.kt / NoteListRemoteViewsFactory.kt — pass widgetId instead of listId to factory
• widget_note_list.xml — header row now has title + chevron ImageView
• AndroidManifest.xml + styles.xml — register new Activity with dialog theme

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