feat: default icon prefix to self-hosted Gitea (CN-reachable)

Switch DEFAULT_ICON_PREFIX from github raw (flaky in CN) to the
self-hosted Gitea at 45.143.131.159:3000/song/a2i. Auto-migrate
existing users whose stored prefix is the old github default; custom
values kept. Icons are already mirrored to Gitea via every push.
CLAUDE.md synced.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/com/a2i/forwarder/store/SettingsStore.kt

@@ -94,7 +94,16 @@ class SettingsStore(
                 _servers.value = list
                 _currentServerId.value = p[K.CURRENT] ?: ID_OFFICIAL
                 currentServer.value = list.firstOrNull { it.id == _currentServerId.value } ?: list.firstOrNull()
-                iconPrefix.value = p[K.ICON_PREFIX] ?: DEFAULT_ICON_PREFIX
+                // 图标前缀：旧 github 默认值自动迁移到 Gitea；用户自定义值保留
+                val storedPrefix = p[K.ICON_PREFIX]
+                iconPrefix.value = when (storedPrefix) {
+                    null -> DEFAULT_ICON_PREFIX
+                    LEGACY_ICON_PREFIX -> {
+                        scope.launch { edit { it[K.ICON_PREFIX] = DEFAULT_ICON_PREFIX } }
+                        DEFAULT_ICON_PREFIX
+                    }
+                    else -> storedPrefix
+                }
                 forwardingEnabled.value = p[K.FWD] ?: true
                 codeEnabled.value = p[K.CODE] ?: true
                 adFilterEnabled.value = p[K.AD] ?: true
@@ -184,6 +193,8 @@ class SettingsStore(
 
     companion object {
         const val ID_OFFICIAL = "official"
-        const val DEFAULT_ICON_PREFIX = "https://raw.githubusercontent.com/lsxf/a2i/main/icons/"
+        // 图标前缀默认走自建 Gitea（国内可达）；github raw 作为备份
+        const val DEFAULT_ICON_PREFIX = "http://45.143.131.159:3000/song/a2i/raw/branch/main/icons/"
+        const val LEGACY_ICON_PREFIX = "https://raw.githubusercontent.com/lsxf/a2i/main/icons/"
     }
 }