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/styles.css

@@ -9192,6 +9192,20 @@ kbd {
   gap: 0.25rem;
 }
 
+/* Show-hidden toggle: a literal `.*` glyph rather than an icon, so its meaning
+ * (dot-prefixed files and folders) survives every skin and font stack. */
+.btn-file-browser-hidden {
+  font-family: var(--font-mono, monospace);
+  font-size: 0.85rem;
+  font-weight: 700;
+  letter-spacing: -0.05em;
+}
+
+.btn-file-browser-hidden.active {
+  color: var(--accent);
+  background: var(--bg-hover);
+}
+
 .file-browser-search {
   padding: 0.4rem;
   border-bottom: 1px solid var(--border);