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/scripts/build-chrome-store.mjs

@@ -76,11 +76,13 @@ function processSidepanelJs(content) {
 function processManifestJson(content) {
   const manifest = JSON.parse(content)
 
-  // Remove localhost from host_permissions
-  if (manifest.host_permissions) {
-    manifest.host_permissions = manifest.host_permissions.filter(
-      perm => !perm.includes('localhost:3000') && !perm.includes('127.0.0.1:3000')
-    )
+  manifest.host_permissions = [
+    'https://memento-note.com/*',
+    'https://www.memento-note.com/*',
+  ]
+
+  if (manifest.content_scripts?.[0]) {
+    manifest.content_scripts[0].matches = ['https://memento-note.com/*', 'https://www.memento-note.com/*']
   }
 
   return JSON.stringify(manifest, null, 2)