feat: chunks recherche (snippets) + script migration

1. Recherche: fetchChunkSnippets() — après le classement RRF existant,
   récupère les passages précis qui matchent depuis NoteEmbeddingChunk.
   Pur affichage, AUCUN changement de classement.

2. Script migration: scripts/migrate-chunk-embeddings.ts
   Indexe toutes les notes existantes en fragments.
   Batch de 10, barre de progression.
   Usage: npx tsx scripts/migrate-chunk-embeddings.ts

3. Memory Echo chunk-level: à faire (US restante)

memento-note/extension/dist-chrome-store/sidepanel.js

@@ -246,6 +246,14 @@ function bindIdleHandlers() {
   document.getElementById('clipSelBtn')?.addEventListener('click', () => void runAnalyze('selection'))
   document.getElementById('clipPageBtn')?.addEventListener('click', () => void runAnalyze('page'))
   document.getElementById('clipLinkBtn')?.addEventListener('click', () => void runAnalyze('link'))
+
+  // Gérer l'erreur de chargement du favicon
+  document.querySelector('.page-favicon')?.addEventListener('error', function() {
+    const fallback = this.getAttribute('data-fallback')
+    if (fallback && this.src !== fallback) {
+      this.src = fallback
+    }
+  })
 }
 
 async function clearSelection() {
@@ -410,7 +418,7 @@ function renderIdle() {
     <div class="page-card">
       <span class="sub">${escapeHtml(t('activePage'))}</span>
       <div class="page-row">
-        <img src="${escapeHtml(pageFavicon)}" alt="" onerror="this.src='https://www.google.com/s2/favicons?domain=google.com&sz=32'" />
+        <img src="${escapeHtml(pageFavicon)}" alt="" class="page-favicon" data-fallback="https://www.google.com/s2/favicons?domain=google.com&sz=32" />
         <div class="page-text">
           <div class="title"${rtlAttrs(pageTitle)}>${escapeHtml(pageTitle || '—')}</div>
           <div class="url">${escapeHtml(pageUrl || '—')}</div>