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>

.changeset/file-viewer-show-hidden.md

@@ -0,0 +1,23 @@
+---
+'aicodeman': patch
+---
+
+The File Viewer can show hidden files and folders.
+
+`GET /api/sessions/:id/files` has always accepted `showHidden=true`, but the panel
+hardcoded `showHidden=false`, so dot-prefixed entries were unreachable from the
+tree: no `.gitignore`, no `.github/`, no `.env.example`, and nothing under them.
+Opening one meant guessing its path.
+
+The panel header gains a `.*` toggle. It re-fetches rather than re-rendering the
+cached tree, because the filtering happens server-side, and it keeps the expanded
+directories so toggling does not collapse the tree you just navigated. The state
+is per-device (its own `codeman:fileBrowserShowHidden` key rather than the
+app-settings object, which is rebuilt from the settings-modal DOM on save and
+would drop a key toggled from outside it), defaults to OFF, and survives a reload.
+
+Generated and version-control directories (`.git`, `node_modules`, `.next`,
+`.venv`, ...) stay excluded either way: that list is about tree size, not about
+hiding dotfiles.
+
+Closes #221.