feat: ring widget style (selectable bars/rings)

Add RingRenderer (circular gauge mirror of BarRenderer), a widget_layout_rings
layout, a Bars/Rings preference + in-app toggle, and ring rendering branch in
ClaudeUsageWidget. Full (4x2) widget honors the chosen style; compact size stays
bars. Phase 1 of porting hamed-elfayome/Claude-Usage-Tracker features.

app/src/main/java/me/khodak/claudeusage/MainActivity.kt

@@ -66,6 +66,7 @@ class MainActivity : AppCompatActivity() {
         }
 
         setupNotificationSettings()
+        setupWidgetStyleSetting()
 
         binding.btnDebug.setOnClickListener {
             if (binding.tvDebugInfo.visibility == android.view.View.GONE) {
@@ -112,6 +113,15 @@ class MainActivity : AppCompatActivity() {
         if (prefs.isNotifyEnabled()) requestNotificationPermission()
     }
 
+    /** Bars ⇄ Rings switch for the home-screen widget; redraws any placed widgets immediately. */
+    private fun setupWidgetStyleSetting() {
+        binding.switchRingStyle.isChecked = prefs.getWidgetStyle() == PreferencesManager.STYLE_RINGS
+        binding.switchRingStyle.setOnCheckedChangeListener { _, checked ->
+            prefs.setWidgetStyle(if (checked) PreferencesManager.STYLE_RINGS else PreferencesManager.STYLE_BARS)
+            ClaudeUsageWidget.notifyDataChanged(this)
+        }
+    }
+
     private fun requestNotificationPermission() {
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.TIRAMISU) return
         if (ContextCompat.checkSelfPermission(this, Manifest.permission.POST_NOTIFICATIONS)