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

tests/frontend/managers/settingsManager.downloadSkipBaseModels.test.js

@@ -72,12 +72,6 @@ vi.mock('../../../static/js/managers/BannerService.js', () => ({
     },
 }));
 
-vi.mock('../../../static/js/components/SidebarManager.js', () => ({
-    sidebarManager: {
-        setSidebarEnabled: vi.fn().mockResolvedValue(),
-    },
-}));
-
 import { SettingsManager } from '../../../static/js/managers/SettingsManager.js';
 import { state } from '../../../static/js/state/index.js';