feat(config): unify common config snippets persistence across all apps

- Add unified `common_config_snippets` structure to MultiAppConfig
- Implement `get_common_config_snippet` and `set_common_config_snippet` commands
- Replace localStorage with config.json persistence for Codex and Gemini
- Auto-migrate legacy `claude_common_config_snippet` to new unified structure
- Deprecate individual API methods in favor of unified interface
- Add automatic migration from localStorage on first load

BREAKING CHANGE: Common config snippets now stored in unified `common_config_snippets` object instead of separate fields

src-tauri/src/app_config.rs

@@ -174,6 +174,39 @@ impl FromStr for AppType {
     }
 }
 
+/// 通用配置片段（按应用分治）
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+pub struct CommonConfigSnippets {
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub claude: Option<String>,
+
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub codex: Option<String>,
+
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub gemini: Option<String>,
+}
+
+impl CommonConfigSnippets {
+    /// 获取指定应用的通用配置片段
+    pub fn get(&self, app: &AppType) -> Option<&String> {
+        match app {
+            AppType::Claude => self.claude.as_ref(),
+            AppType::Codex => self.codex.as_ref(),
+            AppType::Gemini => self.gemini.as_ref(),
+        }
+    }
+
+    /// 设置指定应用的通用配置片段
+    pub fn set(&mut self, app: &AppType, snippet: Option<String>) {
+        match app {
+            AppType::Claude => self.claude = snippet,
+            AppType::Codex => self.codex = snippet,
+            AppType::Gemini => self.gemini = snippet,
+        }
+    }
+}
+
 /// 多应用配置结构（向后兼容）
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct MultiAppConfig {
@@ -188,7 +221,10 @@ pub struct MultiAppConfig {
     /// Prompt 配置（按客户端分治）
     #[serde(default)]
     pub prompts: PromptRoot,
-    /// Claude 通用配置片段（JSON 字符串，用于跨供应商共享配置）
+    /// 通用配置片段（按应用分治）
+    #[serde(default)]
+    pub common_config_snippets: CommonConfigSnippets,
+    /// Claude 通用配置片段（旧字段，用于向后兼容迁移）
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub claude_common_config_snippet: Option<String>,
 }
@@ -209,6 +245,7 @@ impl Default for MultiAppConfig {
             apps,
             mcp: McpRoot::default(),
             prompts: PromptRoot::default(),
+            common_config_snippets: CommonConfigSnippets::default(),
             claude_common_config_snippet: None,
         }
     }
@@ -278,6 +315,13 @@ impl MultiAppConfig {
             updated = true;
         }
 
+        // 迁移通用配置片段：claude_common_config_snippet → common_config_snippets.claude
+        if let Some(old_claude_snippet) = config.claude_common_config_snippet.take() {
+            log::info!("迁移通用配置：claude_common_config_snippet → common_config_snippets.claude");
+            config.common_config_snippets.claude = Some(old_claude_snippet);
+            updated = true;
+        }
+
         if updated {
             log::info!("配置结构已更新（包括 MCP 迁移或 Prompt 自动导入），保存配置...");
             config.save()?;