feat(browse): unify export formats via one matrix; add PDF to Export menu

The Export context-menu offered only md↔docx↔html (a symmetric set), so PDF
— which the server supports only as md→pdf — was missing. The markdown
editor's DOCX/HTML/PDF buttons hardcoded their own list, so the two could
drift.

Introduce a single source of truth in download.js: EXPORT_MATRIX mirrors
zddc/internal/convert.Convert() exactly (md→docx|html|pdf, docx→md|html,
html→md|docx), exposed as download.exportTargets(ext) + download.convertUrl().
The Export submenu and the editor's buttons both consume it, so a .md file now
offers PDF in the menu and the two surfaces can never disagree. PDF stays
markdown-only (no docx→pdf / html→pdf path exists server-side).

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

browse/js/download.js

@@ -24,6 +24,32 @@
 
     function events() { return window.app.modules.events; }
 
+    // Canonical document-conversion matrix — mirrors zddc/internal/convert
+    // Convert(): which target formats a given source extension can be exported
+    // to. PDF is markdown-only (md→pdf) because the server has no docx→pdf /
+    // html→pdf path. This is the SINGLE source of truth for both the Export
+    // context-menu (download.exportTargets) and the markdown editor's
+    // DOCX/HTML/PDF buttons (preview-markdown.js), so the two never drift.
+    var EXPORT_MATRIX = {
+        md: ['docx', 'html', 'pdf'],
+        docx: ['md', 'html'],
+        html: ['md', 'docx']
+    };
+
+    // exportTargets returns the formats a file of extension `ext` can be
+    // exported to (excludes the source format itself), or [] if `ext` is not a
+    // convertible source. Case-insensitive.
+    function exportTargets(ext) {
+        return EXPORT_MATRIX[String(ext || '').toLowerCase()] || [];
+    }
+
+    // convertUrl maps a source path/URL to its sibling virtual-conversion URL
+    // (foo.md → foo.pdf). zddc-server recognises the sibling-extension pattern
+    // and converts on the fly. Shared by exportFile and the editor buttons.
+    function convertUrl(path, fmt) {
+        return String(path || '').replace(/\.[^./]+$/, '') + '.' + fmt;
+    }
+
     function isHiddenName(name) {
         return name.length === 0 || name[0] === '.' || name[0] === '_';
     }
@@ -202,8 +228,8 @@
             events().statusError('No path for ' + node.name);
             return;
         }
-        var url = path.replace(/\.[^./]+$/, '') + '.' + fmt;
+        var url = convertUrl(path, fmt);
-        var name = node.name.replace(/\.[^./]+$/, '') + '.' + fmt;
+        var name = convertUrl(node.name, fmt);
         events().statusInfo('Exporting ' + name + '…');
         downloadUrl(name, url);
         setTimeout(function () { events().statusClear(); }, 2500);
@@ -212,6 +238,8 @@
     window.app.modules.download = {
         downloadFile: downloadFile,
         downloadFolder: downloadFolder,
-        exportFile: exportFile
+        exportFile: exportFile,
+        exportTargets: exportTargets,
+        convertUrl: convertUrl
     };
 })();

browse/js/menu-model.js

@@ -47,9 +47,16 @@
     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):
+    // The Export submenu's convertible-format set comes from the download
-    // a file of one of these extensions can be exported as the other two.
+    // module's canonical matrix (download.exportTargets), which mirrors the
-    var EXPORT_FORMATS = ['md', 'docx', 'html'];
+    // server's conversion matrix — the single source of truth shared with the
+    // markdown editor's DOCX/HTML/PDF buttons. exportTargets(ext) returns the
+    // target formats for a source extension (e.g. md → docx, html, pdf), or []
+    // when the extension isn't a convertible source.
+    function exportTargets(ext) {
+        var d = window.app.modules.download;
+        return (d && d.exportTargets) ? d.exportTargets(ext) : [];
+    }
     function cap() { return window.zddc && window.zddc.cap; }
 
     function canVerb(node, verb) {
@@ -181,9 +188,10 @@
             }
         },
         {
-            // Export submenu: a folder offers ".zip" (both modes); a md/docx/html
+            // Export submenu: a folder offers ".zip" (both modes); a convertible
-            // file offers the OTHER two formats (server-side conversion, so
+            // file (md/docx/html) offers its server-side conversion targets —
-            // server mode only). A zip is already an archive — no Export.
+            // md → docx/html/pdf, docx → md/html, html → md/docx (server mode
+            // only). A zip is already an archive — no Export.
             id: 'export', group: 'io', surfaces: ['row'],
             label: 'Export',
             appliesTo: function (ctx) {
@@ -191,7 +199,7 @@
                 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;
+                return isServer() && exportTargets(n.ext).length > 0;
             },
             items: function (ctx) {
                 var n = ctx.node;
@@ -200,8 +208,8 @@
                 if (n.isDir) {
                     return [{ label: '.zip', action: function () { d.downloadFolder(n); } }];
                 }
-                var cur = (n.ext || '').toLowerCase();
+                // exportTargets already excludes the source format.
-                return EXPORT_FORMATS.filter(function (f) { return f !== cur; }).map(function (fmt) {
+                return exportTargets(n.ext).map(function (fmt) {
                     return { label: '.' + fmt, action: function () { d.exportFile(n, fmt); } };
                 });
             }

browse/js/preview-markdown.js

@@ -465,11 +465,18 @@
             // and routes through ServeConverted. Cleaner than the
             // old `?convert=` query form — right-clicking the link
             // gives a sensible "Save as <file>.docx" prompt.
-            var mdUrlBase = node.url.replace(/\.md$/i, '');
+            //
-            ['docx', 'html', 'pdf'].forEach(function (fmt) {
+            // Format set + URL come from the download module's canonical
+            // conversion matrix (download.exportTargets / convertUrl) — the
+            // SAME source of truth the Export context-menu uses, so the
+            // editor's buttons and the menu never offer different formats.
+            var dl = window.app.modules.download;
+            var mdTargets = (dl && dl.exportTargets) ? dl.exportTargets('md') : ['docx', 'html', 'pdf'];
+            mdTargets.forEach(function (fmt) {
                 var a = document.createElement('a');
                 a.className = 'btn btn-sm btn-secondary md-shell__download';
-                a.href = mdUrlBase + '.' + fmt;
+                a.href = (dl && dl.convertUrl) ? dl.convertUrl(node.url, fmt)
+                    : node.url.replace(/\.md$/i, '') + '.' + fmt;
                 // target=_blank: clicks open in a new tab. The server
                 // sends Content-Disposition: inline, so the new tab
                 // either renders (HTML → web page; PDF → browser's