feat: persist workspace tree expanded state across refreshes

Store expanded directory paths in localStorage keyed by workspace path
(key: 'hermes-webui-expanded:{workspacePath}'). On root load (loadDir('.')),
restore the saved set for the current workspace and pre-fetch dir contents
for any restored expanded directories so the tree renders fully on first
paint without requiring a second click to expand.

Saves on every expand/collapse toggle. Switching workspaces automatically
picks up that workspace's own saved state. Per-workspace (not per-session)
so the same tree state is shared across sessions using the same workspace,
which is the natural expectation.

static/ui.js

@@ -919,9 +919,11 @@ function _renderTreeItems(container, entries, depth){
         e.stopPropagation();
         if(S._expandedDirs.has(item.path)){
           S._expandedDirs.delete(item.path);
+          if(typeof _saveExpandedDirs==='function')_saveExpandedDirs();
           renderFileTree();
         }else{
           S._expandedDirs.add(item.path);
+          if(typeof _saveExpandedDirs==='function')_saveExpandedDirs();
           // Fetch children if not cached
           if(!S._dirCache[item.path]){
             try{