i18n: complete remaining roadmap tasks

JS files:
- ai.js: add t() import, replace 10 hardcoded French strings
- sync.js: add t() import, replace 20 hardcoded French strings (toasts, status labels, events, AI panel)
- graph.js: add t() import, replace 6 hardcoded French strings (tooltips, badges)

Locales:
- +36 new keys for ai.js, sync.js, graph.js
- 1206 keys in each locale file (perfectly synchronized)
- 0 French characters remaining in en.json

Documentation:
- README.fr.md created (French version)
- ROADMAP.md point #63 updated to 100% complete

frontend/js/ai.js

@@ -9,6 +9,7 @@
    - AI Toolbox (to list, to table, frontmatter, to canvas)
 */
 import { api } from './auth.js';
+import { t } from './i18n.js';
 
 // ── API call helper ──
 async function aiAction(endpoint, text, extra = {}) {
@@ -270,7 +271,7 @@ export async function createAIToolbar(container, getEditorView) {
       replaceSelection(v, result, 'append');
       showToast(t('ai.completion_added'), 'success');
     } catch (e) {
-      showToast('AI: ' + (String(e.message || e).includes('401') ? 'clé API invalide' : e.message), 'error');
+      showToast('AI: ' + (String(e.message || e).includes('401') ? t('ai.error_invalid_key') : e.message), 'error');
     } finally {
       aiBtn.innerHTML = origHTML;
       aiBtn.disabled = false;
@@ -278,13 +279,13 @@ export async function createAIToolbar(container, getEditorView) {
   });
 
   // ── Edit menu ──
-  const editBtn = createDropdownBtn('Éditer', [
-    { label: '🪄 Improve writing', action: () => action('improve'), hint: 'Améliore la qualité du texte' },
+  const editBtn = createDropdownBtn(t('ai.edit'), [
+    { label: '🪄 Improve writing', action: () => action('improve'), hint: t('ai.hint_improve') },
     { label: '🔤 Fix spelling & grammar', action: () => action('fix-spelling'), hint: 'Corrige les fautes' },
     { label: '📏 Make shorter', action: () => action('make-shorter'), hint: 'Rend le texte plus concis' },
-    { label: '📐 Make longer', action: () => action('make-longer'), hint: 'Ajoute des détails' },
+    { label: '📐 Make longer', action: () => action('make-longer'), hint: t('ai.hint_add_details') },
     { label: '📋 Simplify language', action: () => action('simplify'), hint: 'Simplifie le langage' },
-  ], 'Modifie le texte sélectionné');
+  ], t('ai.hint_edit_selection'));
 
   // ── Tone menu ──
   const toneBtn = createDropdownBtn('Ton', [
@@ -300,14 +301,14 @@ export async function createAIToolbar(container, getEditorView) {
     { label: '🇩🇪 German', action: () => action('translate', { target_lang: 'German' }) },
     { label: '🇫🇷 French', action: () => action('translate', { target_lang: 'French' }) },
     { label: '🇪🇸 Spanish', action: () => action('translate', { target_lang: 'Spanish' }) },
-  ], 'Traduit le texte sélectionné');
+  ], t('ai.hint_translate'));
 
   // ── Generate menu ──
-  const genBtn = createDropdownBtn('Générer', [
+  const genBtn = createDropdownBtn(t('ai.generate'), [
     { label: 'ℹ️ Explain this', action: () => action('explain', {}, 'append') },
     { label: '📝 Summarize', action: () => action('summarize', {}, 'append') },
     { label: '✏️ Continue writing', action: () => action('continue', {}, 'append') },
-  ], 'Génère du contenu à partir de la sélection');
+  ], t('ai.hint_generate'));
 
   // ── Custom Rewrite ──
   const rewriteBtn = document.createElement('button');
@@ -327,19 +328,19 @@ export async function createAIToolbar(container, getEditorView) {
     const v = ev();
     if (!v) return;
     const text = getSelection(v);
-    const instruction = prompt('Instruction de réécriture :', '');
+    const instruction = prompt(t('ai.rewrite_prompt'), '');
     if (!instruction) return;
     try {
       const result = await aiAction('rewrite', text, { instruction });
       replaceSelection(v, result);
       showToast(t('ai.text_rewritten'), 'success');
     } catch (e) {
-      showToast('AI: ' + (String(e.message || e).includes('401') ? 'clé API invalide' : e.message), 'error');
+      showToast('AI: ' + (String(e.message || e).includes('401') ? t('ai.error_invalid_key') : e.message), 'error');
     }
   });
 
   // ── Toolbox menu ──
-  const toolboxBtn = createDropdownBtn('🧰 Boîte', [
+  const toolboxBtn = createDropdownBtn(t('ai.toolbox'), [
     { label: '📋 Convert to list', action: () => action('to-list') },
     { label: '📊 Convert to table', action: () => action('to-table') },
     { label: '⚙️ Generate frontmatter', action: () => action('frontmatter', {}, 'before') },