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

static/js/recipes.js

@@ -95,8 +95,7 @@ class RecipeManager {
     async _initSidebar() {
         try {
             sidebarManager.setHostPageControls(this.pageControls);
-            const shouldShowSidebar = state?.global?.settings?.show_folder_sidebar !== false;
-            await sidebarManager.setSidebarEnabled(shouldShowSidebar);
+            await sidebarManager.initialize(this.pageControls);
         } catch (error) {
             console.error('Failed to initialize recipe sidebar:', error);
         }