feat(file-viewer): show hidden files and folders

The tree endpoint has accepted `showHidden=true` since it was written; the
panel hardcoded `showHidden=false`, so dot-prefixed entries were unreachable
from the File Viewer and opening one meant guessing its path.

Adds a `.*` toggle to the panel header. It re-fetches instead of re-rendering
the cached tree (the filtering is server-side), preserves the expanded
directories so toggling does not collapse the tree, and persists per-device to
its own `codeman:fileBrowserShowHidden` key. That key is deliberately not part
of the app-settings object, which `saveAppSettings()` rebuilds from the
settings-modal DOM and would drop it on the next save.

Default is OFF, so an untouched install behaves exactly as before.

Closes #221

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -615,6 +615,11 @@ class CodemanApp {
     this.fileBrowserFilter = '';
     this.fileBrowserAllExpanded = false;
     this.fileBrowserDragListeners = null;
+    // Show hidden (dot-prefixed) files and folders in the File Viewer tree.
+    // Per-device, persisted to its own localStorage key by panels-ui.js. Safe to
+    // call a mixin method here: instantiation is deferred to DOMContentLoaded,
+    // so every module's Object.assign has already run.
+    this.fileBrowserShowHidden = this._loadFileBrowserShowHidden?.() ?? false;
     this.filePreviewContent = '';
 
     // Toast container cache (methods in panels-ui.js)