feat(explorer): add Open action to file/folder context menu

Coffee CLI ships no in-app previewers (markdown / pdf / image / etc.),
so before this commit the user's only path from Explorer to a file's
content was Reveal in File Explorer → double-click in the OS file
manager. Add a top-of-menu Open entry that hands the absolute path
straight to the existing open_url command (cmd /C start "" / open /
xdg-open) — files launch in their OS-default app, folders launch in
the OS file manager, file-type-without-default triggers the OS picker.
We don't track defaults or MIME mappings ourselves; the system owns
the whole flow.

Translation key menu.open added to all 11 i18n bundles.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: claude

src-ui/src/components/left/Explorer.tsx

@@ -128,6 +128,19 @@ function ContextMenu({ menu, onClose }: { menu: CtxMenuState; onClose: () => voi
     }
   };
 
+  // Hand the path off to the OS default opener — Notepad / browser / image
+  // viewer / file manager, whichever the user already configured. We don't
+  // ship in-app previewers (memory: "no in-app file viewers"), so this is
+  // the user's only one-click path from Explorer to the file's content.
+  const handleOpen = async () => {
+    onClose();
+    try {
+      await commands.openUrl(menu.absolutePath);
+    } catch (e) {
+      console.error('[Explorer] open failed:', e);
+    }
+  };
+
   const canPaste = !!fsClipboard;
 
   // Smart menu positioning to prevent off-screen clipping
@@ -149,6 +162,16 @@ function ContextMenu({ menu, onClose }: { menu: CtxMenuState; onClose: () => voi
 
   return createPortal(
     <div className="ctx-menu" ref={menuRef} style={style}>
+      {/* Primary action: open in OS default — most-used, sits at the top */}
+      <button className="ctx-menu-item" onClick={handleOpen}>
+        <svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <path d="M15 3h6v6"/>
+          <path d="M10 14 21 3"/>
+          <path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/>
+        </svg>
+        {t('menu.open' as any)}
+      </button>
+      <div className="ctx-menu-divider" />
       {/* Path copy group */}
       <button className="ctx-menu-item" onClick={() => copyPath(menu.absolutePath)}>
         <svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">

src-ui/src/i18n/de.ts

@@ -15,6 +15,7 @@ export const de = {
   'menu.rename': 'Umbenennen',
   'menu.delete': 'Löschen',
   'menu.show_in_folder': 'Im Explorer anzeigen',
+  'menu.open': 'Öffnen',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'Desktop',

src-ui/src/i18n/en.ts

@@ -16,6 +16,7 @@ export const en = {
   'menu.rename': 'Rename',
   'menu.delete': 'Delete',
   'menu.show_in_folder': 'Reveal in File Explorer',
+  'menu.open': 'Open',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'Desktop',

src-ui/src/i18n/es.ts

@@ -15,6 +15,7 @@ export const es = {
   'menu.rename': 'Renombrar',
   'menu.delete': 'Eliminar',
   'menu.show_in_folder': 'Mostrar en el explorador',
+  'menu.open': 'Abrir',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'Escritorio',

src-ui/src/i18n/fr.ts

@@ -15,6 +15,7 @@ export const fr = {
   'menu.rename': 'Renommer',
   'menu.delete': 'Supprimer',
   'menu.show_in_folder': 'Afficher dans l\u2019explorateur',
+  'menu.open': 'Ouvrir',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'Bureau',

src-ui/src/i18n/ja.ts

@@ -15,6 +15,7 @@ export const ja = {
   'menu.rename': '名前を変更',
   'menu.delete': '削除',
   'menu.show_in_folder': 'エクスプローラーで表示',
+  'menu.open': '開く',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'デスクトップ',

src-ui/src/i18n/ko.ts

@@ -15,6 +15,7 @@ export const ko = {
   'menu.rename': '이름 바꾸기',
   'menu.delete': '삭제',
   'menu.show_in_folder': '파일 탐색기에서 열기',
+  'menu.open': '열기',
 
   // Drive kinds (Quick Access)
   'drive.desktop': '바탕화면',

src-ui/src/i18n/pt.ts

@@ -15,6 +15,7 @@ export const pt = {
   'menu.rename': 'Renomear',
   'menu.delete': 'Excluir',
   'menu.show_in_folder': 'Mostrar no explorador',
+  'menu.open': 'Abrir',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'Área de trabalho',

src-ui/src/i18n/ru.ts

@@ -15,6 +15,7 @@ export const ru = {
   'menu.rename': 'Переименовать',
   'menu.delete': 'Удалить',
   'menu.show_in_folder': 'Показать в проводнике',
+  'menu.open': 'Открыть',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'Рабочий стол',

src-ui/src/i18n/vi.ts

@@ -16,6 +16,7 @@ export const vi = {
   'menu.rename': 'Đổi tên',
   'menu.delete': 'Xóa',
   'menu.show_in_folder': 'Hiển thị trong File Explorer',
+  'menu.open': 'Mở',
 
   // Drive kinds (Quick Access)
   'drive.desktop': 'Màn hình nền',