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/de.json

@@ -956,10 +956,7 @@
     },
     "sidebar": {
         "modelRoot": "Stammverzeichnis",
-        "moreOptions": "Weitere Optionen",
         "collapseAll": "Alle Ordner einklappen",
-        "pinSidebar": "Sidebar anheften",
-        "unpinSidebar": "Sidebar lösen",
         "hideOnThisPage": "Seitenleiste auf dieser Seite ausblenden",
         "showSidebar": "Seitenleiste anzeigen",
         "sidebarHiddenNotification": "Seitenleiste auf der Seite {page} ausgeblendet",