fix(browse): dblclick rescopes client-side instead of hard nav

Hard navigation to /<path>/ lets zddc-server's auto-serve kick in for
canonical folders — /archive/ swaps in the archive tool, /staging/ the
transmittal tool, etc. — so double-clicking a canonical folder from
inside browse silently swapped the user out of browse, contrary to
what they expected.

Fix: client-side rescope. navigateIntoFolder() now fetches the new
directory listing via the loader, calls tree.setRoot() with virtual
canonicals re-applied, and pushes the new URL via history.pushState.
The page never reloads. A subsequent reload still works (browse loads
itself at the new URL since trailing-slash → ServeDirectory → embedded
browse SPA).

Side effects:
  - augmentRoot (the canonical-folder injection helper) exposed via
    window.app.modules so events.js can re-apply it on rescope.
  - popstate handler: back/forward in the browser triggers the same
    rescope path with the historic URL.
  - Selection + preview reset on rescope; the previous file's preview
    isn't carried into the new scope.

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

browse/js/app.js

@@ -49,6 +49,9 @@
         return augmented;
     }
 
+    // Expose for events.js's client-side rescope on dblclick.
+    window.app.modules.augmentRoot = withVirtualCanonicals;
+
     async function bootstrap() {
         events.init();
 
@@ -67,6 +70,27 @@
                 + ' from ' + detected.path);
         }
         // Else: empty state stays visible; user can click Select Directory.
+
+        // Browser back / forward: client-side rescope when the URL
+        // changes via popstate. We can't tell server-vs-fs mode from
+        // popstate alone, so only honor it in server mode.
+        window.addEventListener('popstate', async function () {
+            if (window.app.state.source !== 'server') return;
+            var path = location.pathname;
+            if (!path.endsWith('/')) path += '/';
+            try {
+                var es = await loader.fetchServerChildren(path);
+                window.app.state.currentPath = path;
+                window.app.state.selectedId = null;
+                window.app.state.lastPreviewedNodeId = null;
+                tree.setRoot(withVirtualCanonicals(es, path));
+                tree.render();
+                var previewBody = document.getElementById('previewBody');
+                if (previewBody) previewBody.innerHTML = '';
+                var previewTitle = document.getElementById('previewTitle');
+                if (previewTitle) previewTitle.textContent = 'No file selected';
+            } catch (_e) { /* swallow — leave the tree as-is */ }
+        });
     }
 
     if (document.readyState === 'loading') {

browse/js/events.js

@@ -311,9 +311,16 @@
 
     async function navigateIntoFolder(node) {
         if (state.source === 'server') {
+            // Rescope client-side rather than hard-navigating. A hard
+            // nav would let zddc-server's auto-serve kick in and swap
+            // us out of browse for canonical folders (e.g. /archive/
+            // → archive tool, /staging/ → transmittal). Staying in
+            // browse is what the user asked for; pushState keeps the
+            // URL bar accurate so a reload would re-load browse at the
+            // new scope.
             var url = window.app.modules.tree.pathFor(node);
             if (!url.endsWith('/')) url += '/';
-            window.location.assign(url);
+            await rescopeServer(url, node.name);
             return;
         }
         if (state.source === 'fs') {
@@ -333,6 +340,47 @@
         }
     }
 
+    // Client-side rescope for server mode. Updates the URL via
+    // history.pushState, fetches the new directory listing, and
+    // re-renders the tree from scratch. Page DOES NOT reload.
+    async function rescopeServer(url, displayName) {
+        var entries;
+        try {
+            entries = await loader.fetchServerChildren(url);
+        } catch (e) {
+            statusError('Failed to enter ' + displayName + ': ' + (e.message || e));
+            return;
+        }
+        state.currentPath = url;
+        // Selection / preview belong to the old scope; clear them so
+        // the new root doesn't carry stale highlight state.
+        state.selectedId = null;
+        state.lastPreviewedNodeId = null;
+        // Inject virtual canonical folders at the new scope if it's a
+        // project root. (app.js owns this helper; expose via window.app.)
+        var augment = window.app.modules.augmentRoot;
+        var rootEntries = (typeof augment === 'function')
+            ? augment(entries, url)
+            : entries;
+        tree.setRoot(rootEntries);
+        tree.render();
+        // Reset the preview pane so the user sees an "empty selection"
+        // state at the new scope instead of the previous file.
+        var previewBody = document.getElementById('previewBody');
+        if (previewBody) previewBody.innerHTML = '';
+        var previewTitle = document.getElementById('previewTitle');
+        if (previewTitle) previewTitle.textContent = 'No file selected';
+        var previewMeta = document.getElementById('previewMeta');
+        if (previewMeta) previewMeta.textContent = '';
+        // pushState so the URL bar reflects the new scope. A real
+        // reload would re-load browse at this URL (trailing slash →
+        // ServeDirectory → embedded browse SPA).
+        try {
+            history.pushState({ zddcBrowse: true, path: url }, '', url);
+        } catch (_e) { /* private browsing edge cases */ }
+        statusInfo('Entered ' + displayName);
+    }
+
     // Public API
     window.app.modules.events = {
         init: init,