feat: #152 viewer XLSX — affichage multi-feuilles, édition des cellules et téléchargement des .xlsx

- backend/xlsx_reader.py : rend openpyxl en tableaux HTML (plafond 500x40 par feuille)
- PUT /api/file/{vault}/xlsx/save : service edit_xlsx_cells (backup avant écriture, refs A1 validées)
- frontend : renderXlsxViewer (onglets, contenteditable, sauvegarde par feuille)
- docs : CHANGELOG [Unreleased], Roadmap index #152, archive, README FR/EN, exemple OpenAPI

backend/openapi_docs.py

@@ -181,6 +181,10 @@ _ENDPOINT_EXAMPLES: dict[tuple[str, str], dict[str, Any]] = {
         "request": {"path": "notes/Accueil.md", "content": "# Accueil\n\nMis à jour."},
         "response": {"status": "ok", "vault": "TestVault", "path": "notes/Accueil.md", "size": 26},
     },
+    ("put", "/api/file/{vault_name}/xlsx/save"): {
+        "request": {"sheet": "Budget", "cells": {"B1": "250"}},
+        "response": {"status": "ok", "vault": "TestVault", "path": "data/budget.xlsx", "size": 1},
+    },
     ("post", "/api/search/replace"): {
         "request": {"query": "Python", "replacement": "Python 3", "vault": "all", "dry_run": True},
         "response": {"matches": [{"vault": "TestVault", "path": "note1.md", "title": "Python", "match_count": 3}], "total_matches": 3, "dry_run": True},