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/index.html

@@ -407,6 +407,7 @@
       <div class="file-browser-header">
         <span class="file-browser-title">Files</span>
         <div class="file-browser-actions">
+          <button class="btn-icon-sm btn-file-browser-hidden" onclick="app.toggleFileBrowserHidden()" title="Show hidden files and folders" aria-label="Show hidden files and folders" aria-pressed="false" id="fileBrowserHiddenBtn">.*</button>
           <button class="btn-icon-sm" onclick="app.refreshFileBrowser()" title="Refresh">&#x21BB;</button>
           <button class="btn-icon-sm" onclick="app.toggleFileBrowserExpand()" title="Expand/Collapse All" id="fileBrowserExpandBtn">&#x229E;</button>
           <button class="btn-icon-sm" onclick="app.closeFileBrowserPanel()" title="Close">&times;</button>