feat: troncature annoncee et lecture par fenetres des tableurs #153

- BUG-090 (#153 A8) : render_sheets() expose total_rows/total_cols,
  max_rows/max_cols et truncated ; la visionneuse affiche un bandeau
  « Feuille tronquée » (i18n FR/EN) au lieu de couper en silence, et la
  ligne d'en-têtes devient sticky (top:auto sur les numéros de ligne).
- #153 A9 : GET /api/file/{vault}/xlsx/sheet?sheet&offset&limit sert une
  fenêtre de 1 à 1000 lignes avec les vraies coordonnées A1, has_more de
  pagination et valeurs calculées A12 ; 404 feuille inconnue, 415 non-xlsx.
- Tests : TestXlsxTruncationNotice (4) + TestXlsxSheetWindow (11) avec
  contre-preuves, xlsx-viewer.test.mjs 14/14, E2E 7/7 (fixture
  sample-xlsx-large.xlsx 520 lignes), suite 1417 passed / 6 skipped,
  ruff/mypy 0, i18n parity.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <[email protected]>

frontend/js/viewer.js

@@ -1005,6 +1005,35 @@ export function renderVideoViewer(area, data) {
 // confirmation before retrying with `force: true`. A value starting with "=" or
 // "@" is stored as text unless the user turns the formula toggle on, so a typed
 // `=cmd|…` cannot execute when the file is later opened in Excel.
+// #153 A8 — a sheet bigger than the render caps used to be silently cut: the
+// user saw a short table and no way to tell the rest of the workbook still
+// existed. The backend now reports the real dimensions of every sheet, so the
+// note states exactly what is hidden (and that those cells are not editable
+// here — the workbook itself is untouched). A payload without those fields
+// (older cache) simply shows no note.
+function truncationNote(sheet) {
+  // The cap is the real "shown" figure, not `rows`/`cols`: those are post-trim
+  // (a sparse sheet renders 1x1) while the note must say how far the view
+  // reaches.
+  const cap = { rows: Number(sheet.max_rows) || 0, cols: Number(sheet.max_cols) || 0 };
+  const reasons = [];
+  if (Number(sheet.total_rows) > cap.rows) {
+    reasons.push(t("xlsx.truncated_rows", { shown: cap.rows, total: Number(sheet.total_rows) }));
+  }
+  if (Number(sheet.total_cols) > cap.cols) {
+    reasons.push(t("xlsx.truncated_cols", { shown: cap.cols, total: Number(sheet.total_cols) }));
+  }
+  if (!reasons.length) return "";
+  return `<div class="xlsx-truncated" role="note">
+      <i data-lucide="scissors" class="xlsx-truncated-icon"></i>
+      <div class="xlsx-warning-body">
+        <strong>${escapeHtml(t("xlsx.truncated_title"))}</strong>
+        <span>${escapeHtml(reasons.join(" "))}</span>
+        <span class="xlsx-warning-hint">${escapeHtml(t("xlsx.truncated_hint"))}</span>
+      </div>
+    </div>`;
+}
+
 export function renderXlsxViewer(area, data) {
   const sheets = data.xlsx_sheets || [];
   const lossy = data.xlsx_lossy_features || [];
@@ -1019,7 +1048,7 @@ export function renderXlsxViewer(area, data) {
       ).join("")}</div>`
     : "";
   const panels = sheets.map((s, i) =>
-    `<div class="xlsx-panel" data-sheet="${i}"${i === 0 ? "" : ' style="display:none"'}>${s.html}</div>`
+    `<div class="xlsx-panel" data-sheet="${i}"${i === 0 ? "" : ' style="display:none"'}>${truncationNote(s)}${s.html}</div>`
   ).join("");
   const lossWarning = lossy.length
     ? `<div class="xlsx-warning" role="note">