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

templates/components/folder_sidebar.html

@@ -1,6 +1,3 @@
-<!-- Hover detection area -->
-<div class="sidebar-hover-area" id="sidebarHoverArea"></div>
-
 <!-- Folder Navigation Sidebar -->
 <div class="folder-sidebar" id="folderSidebar">
     <div class="sidebar-header" id="sidebarHeader">
@@ -15,23 +12,9 @@
             <button class="sidebar-action-btn" id="sidebarCollapseAll" title="{{ t('sidebar.collapseAll') }}">
                 <i class="fas fa-compress-alt"></i>
             </button>
-            <button class="sidebar-action-btn" id="sidebarPinToggle" title="{{ t('sidebar.unpinSidebar') }}">
-                <i class="fas fa-thumbtack"></i>
+            <button class="sidebar-action-btn" id="sidebarHideToggle" title="{{ t('sidebar.hideOnThisPage') }}">
+                <i class="fas fa-chevron-left"></i>
             </button>
-            <button class="sidebar-action-btn" id="sidebarMoreToggle" title="{{ t('sidebar.moreOptions') }}">
-                <i class="fas fa-ellipsis-v"></i>
-            </button>
-        </div>
-        <!-- Dropdown menu for more options -->
-        <div class="sidebar-more-dropdown" id="sidebarMoreDropdown">
-            <div class="sidebar-dropdown-item" data-action="toggle-pin">
-                <i class="fas fa-thumbtack"></i>
-                <span id="sidebarMorePinLabel">{{ t('sidebar.pinSidebar') }}</span>
-            </div>
-            <div class="sidebar-dropdown-item" data-action="toggle-hide">
-                <i class="fas fa-eye-slash"></i>
-                <span>{{ t('sidebar.hideOnThisPage') }}</span>
-            </div>
         </div>
     </div>
     <div class="sidebar-content">