feat(chess): add dismiss/clear-all for finished games in lobby

Persist dismissed game IDs locally (SharedPreferences on Android,
java.util.prefs on Desktop) so completed games can be permanently
hidden from the chess lobby. Adds per-game dismiss button and
"Clear all" action when 2+ finished games exist.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

commons/src/androidMain/kotlin/com/vitorpamplona/amethyst/commons/chess/ChessDismissedGamesStorage.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.commons.chess
+
+import android.content.Context
+import android.content.SharedPreferences
+
+actual class ChessDismissedGamesStorage private actual constructor() {
+    private var prefs: SharedPreferences? = null
+
+    actual companion object {
+        private const val PREFS_NAME = "chess_dismissed_games"
+
+        private fun prefsKey(userPubkey: String) = "dismissed_$userPubkey"
+
+        actual fun create(context: Any?): ChessDismissedGamesStorage {
+            val storage = ChessDismissedGamesStorage()
+            val ctx =
+                context as? Context
+                    ?: throw IllegalArgumentException("Android context required")
+            storage.prefs = ctx.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE)
+            return storage
+        }
+    }
+
+    // getStringSet returns a live reference to the internal set — must copy defensively
+    actual fun load(userPubkey: String): Set<String> = prefs?.getStringSet(prefsKey(userPubkey), null)?.toHashSet() ?: emptySet()
+
+    actual fun save(
+        userPubkey: String,
+        ids: Set<String>,
+    ) {
+        prefs?.edit()?.putStringSet(prefsKey(userPubkey), ids)?.apply()
+    }
+}