chore: remove MCP preview labels (#790)

MCP server is no longer in preview. Remove "(Preview)" headings from
READMEs, the purple PREVIEW badge from the UI, and the preview i18n keys.

lib/i18n/dictionaries/en.json

@@ -76,8 +76,7 @@
         "creativeDescription": "Draw something fun and creative",
         "cachedNote": "Examples are cached for instant response",
         "mcpServer": "MCP Server",
-        "mcpDescription": "Use in Claude Desktop, VS Code & Cursor",
-        "preview": "PREVIEW"
+        "mcpDescription": "Use in Claude Desktop, VS Code & Cursor"
     },
     "settings": {
         "title": "Settings",

lib/i18n/dictionaries/ja.json

@@ -76,8 +76,7 @@
         "creativeDescription": "楽しくてクリエイティブなものを描く",
         "cachedNote": "例はキャッシュされ、即座に応答します",
         "mcpServer": "MCP サーバー",
-        "mcpDescription": "Claude Desktop、VS Code、Cursor で使用",
-        "preview": "プレビュー"
+        "mcpDescription": "Claude Desktop、VS Code、Cursor で使用"
     },
     "settings": {
         "title": "設定",

lib/i18n/dictionaries/zh-Hant.json

@@ -76,8 +76,7 @@
         "creativeDescription": "繪製有趣且富有創意的內容",
         "cachedNote": "範例已快取，可即時回應",
         "mcpServer": "MCP 伺服器",
-        "mcpDescription": "在 Claude Desktop、VS Code 和 Cursor 中使用",
-        "preview": "預覽"
+        "mcpDescription": "在 Claude Desktop、VS Code 和 Cursor 中使用"
     },
     "settings": {
         "title": "設定",

lib/i18n/dictionaries/zh.json

@@ -76,8 +76,7 @@
         "creativeDescription": "绘制有趣且富有创意的内容",
         "cachedNote": "示例已缓存，可即时响应",
         "mcpServer": "MCP 服务器",
-        "mcpDescription": "在 Claude Desktop、VS Code 和 Cursor 中使用",
-        "preview": "预览"
+        "mcpDescription": "在 Claude Desktop、VS Code 和 Cursor 中使用"
     },
     "settings": {
         "title": "设置",