feat(browse): persist selection + show-hidden in URL

The browse SPA's URL bar now reflects the currently-selected node and
the show-hidden toggle, so:

  - bookmarking / copy-pasting the URL re-opens the same view
  - reload (forced by the admin-mode toggle, which has to reload to
    pick up the elevation cookie) lands the user back on the same
    selection with intermediates expanded
  - browser back/forward walks history correctly, re-applying both
    the scope AND the file/hidden state at each step

Implementation:
  events.js: syncURLToSelection() — builds <scope>/?file=<rel>&hidden=1
    via URLSearchParams (with %2F → '/' so the URL bar reads cleanly)
    and history.replaceState's it. Called from every selectedId set
    site (single-click, arrow-key nav, right-click), from the show-
    hidden toggle, and after rescopeServer's scope pushState so the
    new scope keeps the hidden flag.
  app.js bootstrap: reads ?hidden=1 in addition to the existing
    auto-flip-on-dotfile logic, so an explicit hidden toggle survives
    reload.
  app.js popstate: re-walks ?file= via openDeepLink so back/forward
    restore not just the scope but the selection + expansion path.
    Also re-applies hidden=1.

Choice: replaceState (not pushState) on selection changes — the only
"intentional" navigation step is the scope rescope (already pushState).
A long click sequence shouldn't pollute history.

What this doesn't cover: sibling folders the user expanded that aren't
on the path-to-selection. Persisting that needs sessionStorage; for
"as much as possible without overcomplicating" the URL-only state
captures scope, selected node, and the path-to-selection (auto-
expanded by the deep-link walker) — the most common case.

FS-API mode (offline / file://) is a no-op — no shareable URL.

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

browse/js/app.js

@@ -81,6 +81,10 @@
         // initial listing fetch so dotfiles appear in the tree.
         var qs = new URLSearchParams(location.search);
         var deepFile = qs.get('file');
+        // Explicit ?hidden=1 in the URL: restore the show-hidden toggle
+        // on reload (the URL is the persistence layer for this flag —
+        // see events.js syncURLToSelection).
+        if (qs.get('hidden') === '1') state.showHidden = true;
         if (deepFile) {
             var segs = deepFile.split('/').filter(Boolean);
             for (var si = 0; si < segs.length; si++) {
@@ -125,6 +129,9 @@
             if (window.app.state.source !== 'server') return;
             var path = location.pathname;
             if (!path.endsWith('/')) path += '/';
+            var popQS = new URLSearchParams(location.search);
+            if (popQS.get('hidden') === '1') window.app.state.showHidden = true;
+            else window.app.state.showHidden = false;
             try {
                 var es = await loader.fetchServerChildren(path);
                 window.app.state.currentPath = path;
@@ -138,6 +145,10 @@
                 if (previewTitle) previewTitle.textContent = 'No file selected';
                 // Reapply view mode for the new URL (incoming/ → grid, etc).
                 if (events.applyResolvedViewMode) events.applyResolvedViewMode();
+                // Re-walk ?file= so back/forward restores selection +
+                // expansion, not just scope.
+                var popFile = popQS.get('file');
+                if (popFile) await openDeepLink(popFile);
             } catch (_e) { /* swallow — leave the tree as-is */ }
         });
     }

browse/js/events.js

@@ -87,6 +87,49 @@
         }
     }
 
+    // syncURLToSelection reflects the current scope + selected node +
+    // show-hidden flag into the URL bar via history.replaceState, so:
+    //   - bookmarks / copy-paste of the URL re-open the same view
+    //   - reload (e.g. after toggling admin mode, which forces a hard
+    //     reload to pick up the elevated cookie) lands the user back
+    //     on the same selection
+    //
+    // Uses replaceState (not pushState) so a long click sequence doesn't
+    // pollute browser history. Scope changes (rescopeServer) still
+    // pushState — that's the only "intentional" navigation step in the
+    // SPA, and back/forward should walk between scopes, not selections.
+    //
+    // FS-API mode has no shareable URL, so this is a no-op there.
+    function syncURLToSelection() {
+        if (state.source !== 'server') return;
+        var scope = state.currentPath || '/';
+        if (!scope.endsWith('/')) scope += '/';
+
+        var params = new URLSearchParams();
+        var node = state.selectedId != null ? state.nodes.get(state.selectedId) : null;
+        if (node) {
+            var abs = tree.pathFor(node);
+            var prefix = scope.replace(/\/$/, '');
+            var rel = abs;
+            if (prefix && abs.indexOf(prefix + '/') === 0) {
+                rel = abs.slice(prefix.length + 1);
+            }
+            // Directory selections get a trailing slash so the URL
+            // round-trips as a navigable folder reference.
+            if (node.isDir && rel && !rel.endsWith('/')) rel += '/';
+            if (rel) params.set('file', rel);
+        }
+        if (state.showHidden) params.set('hidden', '1');
+
+        // URLSearchParams percent-encodes '/' to %2F; the server doesn't
+        // care, but the URL bar reads better with raw slashes.
+        var qs = params.toString().replace(/%2F/g, '/');
+        var url = scope + (qs ? '?' + qs : '');
+        try {
+            history.replaceState({ zddcBrowse: true, path: url }, '', url);
+        } catch (_e) { /* private browsing edge cases */ }
+    }
+
     async function refreshListing() {
         // Snapshot expanded paths + selection BEFORE setRoot clears the
         // tree, then re-apply after the new root is in place. Keeps
@@ -273,6 +316,7 @@
                 state.selectedId = id;
                 state.lastPreviewedNodeId = id;
                 tree.render(); // refresh selection highlight
+                syncURLToSelection();
                 var p = previewMod();
                 if (p) p.showFilePreview(node);
             });
@@ -396,6 +440,7 @@
                 state.selectedId = nextId;
                 var nextNode = state.nodes.get(nextId);
                 tree.render();
+                syncURLToSelection();
                 // Auto-preview files as the keyboard cursor lands on them
                 // so the right pane keeps up with selection. Folders are
                 // selection-only; their preview is "expand to see inside".
@@ -423,6 +468,7 @@
                     if (!node) return;
                     state.selectedId = id;
                     tree.render();
+                    syncURLToSelection();
                     window.zddc.menu.open({
                         x: e.clientX,
                         y: e.clientY,
@@ -720,7 +766,10 @@
             statusInfo('Deleted ' + node.name);
             // Clear selection / preview when they pointed at the
             // now-gone node, so the right pane doesn't keep a ghost.
-            if (state.selectedId === node.id) state.selectedId = null;
+            if (state.selectedId === node.id) {
+                state.selectedId = null;
+                syncURLToSelection();
+            }
             if (state.lastPreviewedNodeId === node.id) {
                 state.lastPreviewedNodeId = null;
                 var pb = document.getElementById('previewBody');
@@ -978,6 +1027,7 @@
               checked: function () { return !!state.showHidden; },
               action:  function () {
                   state.showHidden = !state.showHidden;
+                  syncURLToSelection();
                   refreshListing();
               } }
         ];
@@ -1022,6 +1072,7 @@
               checked: function () { return !!state.showHidden; },
               action:  function () {
                   state.showHidden = !state.showHidden;
+                  syncURLToSelection();
                   refreshListing();
               } }
         ];
@@ -1129,10 +1180,14 @@
         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).
+        // ServeDirectory → embedded browse SPA). Then immediately
+        // replaceState via syncURLToSelection so the new URL also
+        // carries ?hidden=1 if the toggle is on (selection is null
+        // at the new scope; the query gets only `hidden`).
         try {
             history.pushState({ zddcBrowse: true, path: url }, '', url);
         } catch (_e) { /* private browsing edge cases */ }
+        syncURLToSelection();
         statusInfo('Entered ' + displayName);
         // The new scope may have a different default view (grid inside
         // incoming/, browse elsewhere). Re-resolve from the URL now