refactor(frontend): remove MCP import functionality for v3.7.0

Auto-migration at startup is sufficient for upgrading from v3.6.x.
Manual import adds unnecessary complexity since:
- Gemini MCP support launches with v3.7.0 (no legacy data)
- Existing Claude/Codex MCP configs are auto-migrated on first run
- All MCP management should happen within CC Switch

Changes:
- Remove McpImportDialog component
- Remove "Import" button from UnifiedMcpPanel
- Remove import-related i18n translations (en/zh)
- Simplify user experience with single management interface

Note: Backend import commands (import_mcp_from_*) remain for
backward compatibility but are no longer exposed in UI.

src/i18n/locales/zh.json

@@ -455,16 +455,6 @@
         "claude": "Claude",
         "codex": "Codex",
         "gemini": "Gemini"
-      },
-      "import": {
-        "title": "导入 MCP 服务器",
-        "button": "导入",
-        "description": "从已有的应用配置文件中导入 MCP 服务器。导入的服务器将自动启用对应的应用。",
-        "fromClaudeDesc": "从 ~/.claude/claude.json 或 ~/.claude/settings.json 导入 MCP 服务器",
-        "fromCodexDesc": "从 ~/.codex/config.toml 导入 MCP 服务器",
-        "fromGeminiDesc": "从 Gemini 配置文件导入 MCP 服务器",
-        "success": "成功导入 {{count}} 个服务器",
-        "noServersFound": "未找到可导入的服务器"
       }
     },
     "userLevelPath": "用户级 MCP 配置路径",