refactor(sidebar): remove pin/unpin and global hide, use per-page hide only

- Remove pin/unpin and auto-hide hover mechanism (isPinned, isHovering,
  hoverTimeout, showSidebar/hideSidebar, updateAutoHideState, etc.)
- Remove global show_folder_sidebar setting (SettingsManager,
  PageControls, recipes, backend default)
- Simplify sidebar visibility to a single per-page toggle:
  · Dedicated chevron-left button in header to hide sidebar
  · Edge indicator (chevron-right) to restore when hidden
  · No dropdown, no hover area, no pin button
- Add _migrateOldSettings() to convert old sidebarPinned and
  show_folder_sidebar states to per-page sidebarDisabled
- Fix sidebar flicker on page load: CSS defaults to off-screen,
  JS explicitly sets .visible or .hidden-by-setting
- Remove obsolete CSS classes: auto-hide, hover-active, collapsed
- Remove i18n keys: pinSidebar, unpinSidebar, moreOptions
- Update test mocks for the new initialize() interface

py/services/settings_manager.py

@@ -91,7 +91,6 @@ DEFAULT_SETTINGS: Dict[str, Any] = {
     "autoplay_on_hover": False,
     "display_density": "default",
     "card_info_display": "always",
-    "show_folder_sidebar": True,
     "include_trigger_words": False,
     "compact_mode": False,
     "priority_tags": DEFAULT_PRIORITY_TAG_CONFIG.copy(),