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/style.css

@@ -10967,12 +10967,25 @@ body.desktop-mode .editor-container {
   border-right: 1px solid var(--border-light, var(--border));
   position: sticky;
   left: 0;
-  z-index: 1;
+  /* #153 A8 — `top: auto` is load-bearing: `.csv-table th` pins EVERY `th`
+     at `top: 0`, so a row number left sticky on both axes piles up in the
+     top-left corner instead of tracking its own row. */
+  top: auto;
+  z-index: 2;
 }
 .xlsx-table th.xlsx-corner {
   left: 0;
   top: 0;
-  z-index: 2;
+  z-index: 4;
+}
+/* #153 A8 — the column headers stay visible while the sheet scrolls down.
+   Declared explicitly (and not inherited from `.csv-table th`) so the stacking
+   order is intentional: thead (3) < row numbers (2) < corner (4). */
+.xlsx-table thead th {
+  position: sticky;
+  top: 0;
+  z-index: 3;
+  background: var(--surface);
 }
 .xlsx-table td[contenteditable] {
   cursor: text;
@@ -11051,6 +11064,31 @@ body.desktop-mode .editor-container {
   color: var(--text-secondary);
   opacity: 0.85;
 }
+/* #153 A8 — "feuille tronquée" notice. Deliberately NOT the `.xlsx-warning`
+   look: that one is a data-loss alert, this one only says part of the sheet is
+   out of view. */
+.xlsx-truncated {
+  display: flex;
+  align-items: flex-start;
+  gap: 8px;
+  padding: 8px 10px;
+  margin-bottom: 8px;
+  border: 1px solid var(--border);
+  border-left: 3px solid var(--accent, #4a90d9);
+  border-radius: 4px;
+  background: var(--bg-secondary);
+  color: var(--text-secondary);
+  font-size: 0.82rem;
+  line-height: 1.45;
+}
+.xlsx-truncated-icon {
+  width: 16px;
+  height: 16px;
+  flex: 0 0 auto;
+  margin-top: 1px;
+  color: var(--accent, #4a90d9);
+}
+
 .xlsx-formula-toggle {
   font-family: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;
   font-weight: 600;