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/managers/SettingsManager.js

@@ -15,7 +15,6 @@ import { i18n } from '../i18n/index.js';
 import { configureModelCardVideo } from '../components/shared/ModelCard.js';
 import { validatePriorityTagString, getPriorityTagSuggestionsMap, invalidatePriorityTagSuggestionsCache } from '../utils/priorityTagHelpers.js';
 import { bannerService } from './BannerService.js';
-import { sidebarManager } from '../components/SidebarManager.js';
 
 const VALID_MATURE_BLUR_LEVELS = new Set(['PG13', 'R', 'X', 'XXX']);
 
@@ -884,12 +883,6 @@ export class SettingsManager {
             cardInfoDisplaySelect.value = state.global.settings.card_info_display || 'always';
         }
 
-        const showFolderSidebarCheckbox = document.getElementById('showFolderSidebar');
-        if (showFolderSidebarCheckbox) {
-            const showSidebarSetting = state.global.settings.show_folder_sidebar;
-            showFolderSidebarCheckbox.checked = showSidebarSetting !== false;
-        }
-
         // Set model card footer action
         const modelCardFooterActionSelect = document.getElementById('modelCardFooterAction');
         if (modelCardFooterActionSelect) {
@@ -2949,12 +2942,6 @@ export class SettingsManager {
         const showVersionOnCard = state.global.settings.show_version_on_card !== false;
         document.body.classList.toggle('hide-card-version', !showVersionOnCard);
 
-        const shouldShowSidebar = state.global.settings.show_folder_sidebar !== false;
-        if (sidebarManager && typeof sidebarManager.setSidebarEnabled === 'function') {
-            sidebarManager.setSidebarEnabled(shouldShowSidebar).catch((error) => {
-                console.error('Failed to apply sidebar visibility setting:', error);
-            });
-        }
     }
 }