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/data/PreferencesManager.kt

@@ -132,6 +132,12 @@ class PreferencesManager(context: Context) {
         } catch (e: Exception) { emptyList() }
     }
 
+    // ── Widget style ─────────────────────────────────────────────────────────
+
+    /** Home-screen widget visual style: [STYLE_BARS] (default) or [STYLE_RINGS]. */
+    fun getWidgetStyle(): String = prefs.getString(KEY_WIDGET_STYLE, STYLE_BARS) ?: STYLE_BARS
+    fun setWidgetStyle(style: String) = prefs.edit().putString(KEY_WIDGET_STYLE, style).apply()
+
     // ── Notification settings ────────────────────────────────────────────────
 
     fun isNotifyEnabled(): Boolean = prefs.getBoolean(KEY_NOTIFY_ENABLED, true)
@@ -155,6 +161,10 @@ class PreferencesManager(context: Context) {
         private const val KEY_HISTORY = "usage_history"
         private const val KEY_NOTIFY_ENABLED = "notify_enabled"
         private const val KEY_AUTH_FAILS = "auth_fail_count"
+        private const val KEY_WIDGET_STYLE = "widget_style"
+
+        const val STYLE_BARS = "bars"
+        const val STYLE_RINGS = "rings"
 
         internal const val MIN_HISTORY_GAP_MS = 2 * 60 * 1000L      // collapse readings <2 min apart
         private const val HISTORY_RETENTION_MS = 7 * 24 * 60 * 60 * 1000L  // keep 7 days