feat(bookslm): bouton toggle cacher/afficher la sidebar dans le header

Même mécanisme que le sidebar de navigation gauche :
- Bouton .bookslm-btn-toggle dans le header (icône panel-right-close/open)
- Toggle la classe .hidden sur le panel, persiste dans localStorage
- État restauré au prochain open()
- Le panel reste monté (pas de teardown), juste slid off-screen
- CSS : .bookslm-panel.open.hidden → translateX(100%)
- CSS : .bookslm-btn-toggle suit le pattern .sidebar-toggle-btn

i18n : bookslm.toggle_sidebar ajouté (fr + en)
Import safeCreateIcons ajouté pour rafraîchir l'icône Lucide au toggle.

Vérifié :
- pytest : 504 passed (zéro régression)
- ruff : All checks passed
- frontend unit : 7 passed
- validate-imports : 30 modules / 204 exports
- pane-manager JSDOM : 9/9

frontend/js/bookslm.js

@@ -1,6 +1,7 @@
 // BooksLM — Directory-scoped AI chat panel (style NotebookLM)
 import { t } from './i18n.js';
 import { buildAIPickerUI } from './ai.js';
+import { safeCreateIcons } from './utils.js';
 
 class BooksLM {
   constructor() {
@@ -43,6 +44,14 @@ class BooksLM {
     // Open with animation
     requestAnimationFrame(() => {
       this._panel.classList.add('open');
+      // Apply persisted hidden state (user toggled off in a previous session)
+      try {
+        const savedHidden = localStorage.getItem('obsigate-bookslm-hidden');
+        if (savedHidden === 'true') {
+          this._panel.classList.add('hidden');
+          this._updateToggleIcon();
+        }
+      } catch { /* */ }
     });
     this._isOpen = true;
 
@@ -77,6 +86,39 @@ class BooksLM {
     }
   }
 
+  /**
+   * Toggle the BooksLM sidebar visibility (collapsed/hidden to the right edge
+   * while the panel stays mounted). The state is persisted in localStorage so
+   * the user choice survives page reloads — same UX as the left sidebar toggle.
+   *
+   * The panel stays in the DOM (no teardown); only a CSS transform is applied,
+   * which is what makes this fast and matches the right-sidebar pattern in
+   * ObsiGate.
+   */
+  _toggleSidebar() {
+    if (!this._panel) return;
+    const isHidden = this._panel.classList.toggle('hidden');
+    try {
+      localStorage.setItem('obsigate-bookslm-hidden', isHidden ? 'true' : 'false');
+    } catch { /* */ }
+    this._updateToggleIcon();
+  }
+
+  /**
+   * Swap the toggle icon between "panel-right-close" (visible) and
+   * "panel-right-open" (hidden) so the button visually reflects the state.
+   */
+  _updateToggleIcon() {
+    if (!this._panel) return;
+    const btn = this._panel.querySelector('.bookslm-btn-toggle');
+    if (!btn) return;
+    const isHidden = this._panel.classList.contains('hidden');
+    const icon = btn.querySelector('i[data-lucide]');
+    if (!icon) return;
+    icon.setAttribute('data-lucide', isHidden ? 'panel-right-open' : 'panel-right-close');
+    if (typeof safeCreateIcons === 'function') safeCreateIcons();
+  }
+
   newConversation() {
     this._messages = [];
     this._saveHistory();
@@ -117,6 +159,9 @@ class BooksLM {
     panel.className = 'bookslm-panel';
     panel.innerHTML = `
       <div class="bookslm-header">
+        <button class="bookslm-btn-toggle" title="${t('bookslm.toggle_sidebar')}" aria-label="${t('bookslm.toggle_sidebar')}">
+          <i data-lucide="panel-right-close" style="width:16px;height:16px"></i>
+        </button>
         <span>📚</span>
         <span class="bookslm-title"></span>
         <span class="bookslm-picker-host" style="margin-left:auto"></span>
@@ -144,6 +189,7 @@ class BooksLM {
     panel.querySelector('.bookslm-btn-close').addEventListener('click', () => this.close());
     panel.querySelector('.bookslm-btn-new').addEventListener('click', () => this.newConversation());
     panel.querySelector('.bookslm-btn-export').addEventListener('click', () => this.exportConversation());
+    panel.querySelector('.bookslm-btn-toggle').addEventListener('click', () => this._toggleSidebar());
     panel.querySelector('.bookslm-btn-fullscreen').addEventListener('click', () => {
       this._isFullscreen = !this._isFullscreen;
       panel.classList.toggle('fullscreen', this._isFullscreen);