feat(KB-029): add file browser and editor with CodeMirror 6 integration

- Add server-side file service with list, read, and write operations

- Add file API routes with comprehensive tests

- Create useFileBrowser and useFileEditor hooks for state management

- Add FileBrowser, FileEditor, and FileBrowserModal components

- Integrate Files tab into TaskDetailModal for task worktree browsing

- Add CodeMirror 6 dependencies for syntax highlighting

- Include file browser styles with light/dark theme support

packages/dashboard/app/App.tsx

@@ -223,10 +223,12 @@ function AppInner() {
         onImport={handleGitHubImport}
         tasks={tasks}
       />
-      <TerminalModal
-        isOpen={terminalOpen}
-        onClose={handleTerminalClose}
-      />
+      {terminalOpen && (
+        <TerminalModal
+          isOpen={terminalOpen}
+          onClose={handleTerminalClose}
+        />
+      )}
       <ToastContainer toasts={toasts} onRemove={removeToast} />
     </>
   );

packages/dashboard/app/styles.css

@@ -3497,8 +3497,6 @@ body {
   font-style: italic;
 }
 
-/* Model selector filter styles */
-.model-selector-filter {
   display: flex;
   align-items: center;
   gap: 8px;
@@ -4288,7 +4286,6 @@ html .column.drag-over * {
 }
 
 .file-node-time {
-  font-size: 11px;
   color: var(--text-dim);
   font-family: "SF Mono", Monaco, Consolas, monospace;
   flex-shrink: 0;