feat(mobile): bouton assistant AI central dans la barre du bas

Sur mobile, la pastille flottante etait masquee par la barre d'outils.

- Masque .ai-fab sur mobile
- Ajoute un bouton rond sureleve #mt-ai au centre de la barre (5 items)
- Extrait openAssistant() dans ai-fab.js (reutilise par la FAB et le bouton mobile)
- mobile-toolbar.js : branche le bouton sur openAssistant()

frontend/js/ai-fab.js

@@ -23,7 +23,10 @@ function syncVisibility() {
   _fab.setAttribute('aria-label', label);
 }
 
-async function onClick() {
+// Shared entry point used by the floating FAB and the mobile toolbar button.
+// Reopens a collapsed panel, otherwise opens (or re-focuses) the assistant
+// for the current application context.
+export async function openAssistant() {
   const panel = document.querySelector('.bookslm-panel');
   // Collapsed panel: reveal it instead of reopening.
   if (panel && panel.classList.contains('open') && panel.classList.contains('hidden')) {
@@ -34,6 +37,10 @@ async function onClick() {
   await booksLM.openForCurrentContext();
 }
 
+async function onClick() {
+  await openAssistant();
+}
+
 export function initAIFab() {
   if (_fab) return;
 

frontend/js/mobile-toolbar.js

@@ -2,6 +2,7 @@
    Phases 1-5: fixed bar, vault home, file palette, commands, tab switcher */
 import { state } from './state.js';
 import { TabManager } from './ui.js';
+import { openAssistant } from './ai-fab.js';
 
 // ── Open file palette (search files) ──
 function openFilePalette() {
@@ -109,6 +110,7 @@ function toggleExplorer() {
 export function initMobileToolbar() {
   const explorerBtn = document.getElementById('mt-explorer');
   const searchBtn = document.getElementById('mt-search');
+  const aiBtn = document.getElementById('mt-ai');
   const paletteBtn = document.getElementById('mt-palette');
   const tabsBtn = document.getElementById('mt-tabs');
   const sheetClose = document.getElementById('mt-sheet-close');
@@ -117,6 +119,7 @@ export function initMobileToolbar() {
 
   if (explorerBtn) explorerBtn.addEventListener('click', toggleExplorer);
   if (searchBtn) searchBtn.addEventListener('click', openFilePalette);
+  if (aiBtn) aiBtn.addEventListener('click', () => { openAssistant(); });
   if (paletteBtn) paletteBtn.addEventListener('click', openCommandPalette);
   if (tabsBtn) tabsBtn.addEventListener('click', openTabSwitcher);
   if (sheetClose) sheetClose.addEventListener('click', closeTabSwitcher);