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

frontend/style.css

@@ -10924,6 +10924,69 @@ body.desktop-mode .editor-container {
   background: var(--surface);
 }
 
+/* ── XLSX Viewer ── */
+.xlsx-toolbar {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  margin-bottom: 8px;
+  flex-wrap: wrap;
+}
+.xlsx-toolbar-actions {
+  margin-left: auto;
+  display: flex;
+  gap: 8px;
+}
+.xlsx-tabs {
+  display: flex;
+  gap: 4px;
+  flex-wrap: wrap;
+}
+.xlsx-tab {
+  border: 1px solid var(--border);
+  background: var(--surface);
+  color: var(--text-secondary);
+  border-radius: 4px;
+  padding: 4px 10px;
+  font-size: 0.8rem;
+  cursor: pointer;
+}
+.xlsx-tab.active {
+  background: var(--accent, #4a90d9);
+  border-color: var(--accent, #4a90d9);
+  color: #fff;
+}
+.xlsx-table th.xlsx-corner,
+.xlsx-table th.xlsx-rownum {
+  background: var(--surface);
+  color: var(--text-secondary);
+  font-weight: 400;
+  text-align: right;
+  padding: 6px 8px;
+  border-bottom: 2px solid var(--border);
+  border-right: 1px solid var(--border-light, var(--border));
+  position: sticky;
+  left: 0;
+  z-index: 1;
+}
+.xlsx-table th.xlsx-corner {
+  left: 0;
+  top: 0;
+  z-index: 2;
+}
+.xlsx-table td[contenteditable] {
+  cursor: text;
+  min-width: 40px;
+  white-space: pre-wrap;
+}
+.xlsx-table td[contenteditable]:focus {
+  outline: 2px solid var(--accent, #4a90d9);
+  outline-offset: -2px;
+}
+.xlsx-table td.xlsx-dirty {
+  background: rgba(255, 196, 0, 0.18);
+}
+
 /* ── JSON Viewer ── */
 .json-viewer {
   font-family: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;