feat(browse): double-click a folder to navigate into it

Single-click still expands the folder inline; double-click swaps the
view's root onto the folder. In server mode it loads the folder's URL
(zddc-server returns a fresh browse instance scoped there); in FS-API
mode it re-roots state.rootHandle onto the folder's handle and re-
enumerates. Files and zips are unchanged — files have a single-click
preview, zips have inline JSZip expansion.

Help panel updated to document the new behavior.

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

browse/js/events.js

@@ -202,6 +202,54 @@
                 // Browser handles target=_blank natively for middle
                 // click; don't preventDefault, just don't intercept.
             });
+
+            // Double-click on a folder → "navigate into" it. Distinct
+            // from single-click (which expands inline) so users keep
+            // both UX models. Server mode jumps to the folder URL —
+            // zddc-server returns a fresh browse instance scoped to
+            // that directory. FS-API mode swaps state.rootHandle to
+            // the folder's handle and re-loads, so the user sees
+            // only that subtree at the root level.
+            //
+            // Files: dblclick is left alone — the single-click preview
+            // is already a "look at this file" action; a separate
+            // navigate-into doesn't apply.
+            // ZIPs: skipped too — they're inspected via inline
+            // expansion (JSZip), not navigated into.
+            tbody.addEventListener('dblclick', function (e) {
+                var row = e.target.closest('tr.tree-row');
+                if (!row) return;
+                if (row.dataset.isdir !== 'true') return;
+                var id = parseInt(row.dataset.id, 10);
+                var node = state.nodes.get(id);
+                if (!node) return;
+                e.preventDefault();
+                navigateIntoFolder(node);
+            });
+        }
+    }
+
+    async function navigateIntoFolder(node) {
+        if (state.source === 'server') {
+            var url = window.app.modules.tree.pathFor(node);
+            if (!url.endsWith('/')) url += '/';
+            window.location.assign(url);
+            return;
+        }
+        if (state.source === 'fs') {
+            if (!node.handle || node.handle.kind !== 'directory') return;
+            state.rootHandle = node.handle;
+            state.currentPath = node.handle.name + '/';
+            var raw;
+            try {
+                raw = await loader.fetchFsChildren(node.handle);
+            } catch (e) {
+                statusError('Failed to enter ' + node.name + ': ' + e.message);
+                return;
+            }
+            tree.setRoot(raw);
+            tree.render();
+            statusInfo('Entered ' + node.name);
         }
     }
 

browse/template.html

@@ -126,6 +126,10 @@
             <dl>
                 <dt>Click a folder</dt>
                 <dd>Toggle expand/collapse on that folder.</dd>
+                <dt>Double-click a folder</dt>
+                <dd>Navigate into the folder — it becomes the new root of the
+                view. Server mode loads the folder's URL; local mode re-roots
+                onto that folder's handle.</dd>
                 <dt>Shift-click a folder</dt>
                 <dd>Recursive expand or collapse — applies to the whole subtree.</dd>
                 <dt>Click a file</dt>