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

locales/ko.json

@@ -956,10 +956,7 @@
     },
     "sidebar": {
         "modelRoot": "루트",
-        "moreOptions": "더 많은 옵션",
         "collapseAll": "모든 폴더 접기",
-        "pinSidebar": "사이드바 고정",
-        "unpinSidebar": "사이드바 고정 해제",
         "hideOnThisPage": "이 페이지에서 사이드바 숨기기",
         "showSidebar": "사이드바 표시",
         "sidebarHiddenNotification": "{page} 페이지에서 사이드바가 숨겨져 있습니다",