feat(browse): Export context-menu submenu (folder→.zip, file→other formats)

Add an "Export" item to the row context menu with a submenu:
- a folder offers ".zip" (reuses download.downloadFolder; works offline + server)
- an md/docx/html file offers the OTHER two formats, each triggering a
  server-side conversion download via the new download.exportFile (builds the
  sibling-extension URL and lets the browser pull the converted bytes). File
  conversion is server-only, so it's hidden in offline (FS) mode; a zip is
  already an archive and gets no Export.

menu-model's toMenuItem now passes a descriptor's `items` through as a submenu
(resolved against the captured browse ctx) instead of only emitting action rows.

Verified: 11/11 browse Playwright specs pass (incl. menu/context + Download ZIP);
a logic harness confirms the per-type submenu contents and that clicks route to
download.exportFile / downloadFolder.

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

browse/js/download.js

@@ -183,8 +183,35 @@
         }
     }
 
+    // Export a file converted to another format. Server-only: builds the
+    // sibling-extension URL (foo.docx → foo.md) and lets the browser pull it —
+    // zddc-server recognises the virtual path and converts on the fly, emitting
+    // Content-Disposition. fmt is a bare extension ("md" | "docx" | "html").
+    function exportFile(node, fmt) {
+        if (!node || node.isDir) {
+            events().statusError('Not a file: ' + (node && node.name));
+            return;
+        }
+        if (state.source !== 'server') {
+            events().statusError('Export to .' + fmt + ' needs a server connection');
+            return;
+        }
+        var tree = window.app.modules.tree;
+        var path = tree && tree.pathFor ? tree.pathFor(node) : node.url;
+        if (!path) {
+            events().statusError('No path for ' + node.name);
+            return;
+        }
+        var url = path.replace(/\.[^./]+$/, '') + '.' + fmt;
+        var name = node.name.replace(/\.[^./]+$/, '') + '.' + fmt;
+        events().statusInfo('Exporting ' + name + '…');
+        downloadUrl(name, url);
+        setTimeout(function () { events().statusClear(); }, 2500);
+    }
+
     window.app.modules.download = {
         downloadFile: downloadFile,
-        downloadFolder: downloadFolder
+        downloadFolder: downloadFolder,
+        exportFile: exportFile
     };
 })();

browse/js/menu-model.js

@@ -46,6 +46,10 @@
     function isServer() { return state.source === 'server'; }
     function appliesToFolderLike(node) { return !!(node && (node.isDir || node.isZip)); }
     function appliesToFile(node) { return !!(node && !node.isDir && !node.isZip); }
+
+    // Formats the Export submenu offers for a file (server-side conversion):
+    // a file of one of these extensions can be exported as the other two.
+    var EXPORT_FORMATS = ['md', 'docx', 'html'];
     function cap() { return window.zddc && window.zddc.cap; }
 
     function canVerb(node, verb) {
@@ -176,6 +180,32 @@
                 else d.downloadFile(ctx.node);
             }
         },
+        {
+            // Export submenu: a folder offers ".zip" (both modes); a md/docx/html
+            // file offers the OTHER two formats (server-side conversion, so
+            // server mode only). A zip is already an archive — no Export.
+            id: 'export', group: 'io', surfaces: ['row'],
+            label: 'Export',
+            appliesTo: function (ctx) {
+                var n = ctx.node;
+                if (!n || n.virtual) return false;
+                if (n.isDir) return true;
+                if (n.isZip) return false;
+                return isServer() && EXPORT_FORMATS.indexOf((n.ext || '').toLowerCase()) !== -1;
+            },
+            items: function (ctx) {
+                var n = ctx.node;
+                var d = window.app.modules.download;
+                if (!d) return [];
+                if (n.isDir) {
+                    return [{ label: '.zip', action: function () { d.downloadFolder(n); } }];
+                }
+                var cur = (n.ext || '').toLowerCase();
+                return EXPORT_FORMATS.filter(function (f) { return f !== cur; }).map(function (fmt) {
+                    return { label: '.' + fmt, action: function () { d.exportFile(n, fmt); } };
+                });
+            }
+        },
 
         // ── create (folder rows + pane; NOT file rows) ──
         // Create actions are HIDDEN unless the user can create here (the
@@ -361,7 +391,7 @@
     }
 
     function toMenuItem(d, ctx) {
-        return {
+        var item = {
             label: resolve(d.label, ctx),
             accel: d.accel,
             danger: d.danger,
@@ -370,9 +400,16 @@
             disabled: function () { return !resolveBool(d.enabled, ctx, true); },
             tooltip: function () {
                 return resolveBool(d.enabled, ctx, true) ? '' : (resolve(d.tooltip, ctx) || '');
-            },
-            action: function () { if (d.action) d.action(ctx); }
+            }
         };
+        // A descriptor with `items` becomes a submenu (resolved against the
+        // captured browse ctx); otherwise it's a normal action row.
+        if (d.items) {
+            item.items = function () { return resolve(d.items, ctx); };
+        } else {
+            item.action = function () { if (d.action) d.action(ctx); };
+        }
+        return item;
     }
 
     function project(surface, ctx) {