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]>

backend/schemas.py

@@ -286,7 +286,12 @@ class FileContentResponse(BaseModel):
     is_csv: bool | None = Field(default=None, description="True for CSV files")
     is_xlsx: bool | None = Field(default=None, description="True for Excel .xlsx files")
     xlsx_sheets: list[dict[str, Any]] | None = Field(
-        default=None, description="Rendered xlsx sheets [{name, html}]"
+        default=None,
+        description=(
+            "Rendered xlsx sheets [{name, html, rows, cols, total_rows, "
+            "total_cols, max_rows, max_cols, truncated}] — `truncated` is true "
+            "when the sheet exceeds the 500x40 render caps (#153 A8)"
+        ),
     )
     xlsx_lossy_features: list[str] | None = Field(
         default=None,
@@ -305,6 +310,32 @@ class FileContentResponse(BaseModel):
     image_mime: str | None = Field(default=None, description="MIME type for image files")
 
 
+class XlsxSheetWindowResponse(BaseModel):
+    """One window of rows of a single .xlsx sheet (lazy loading, #153 A9).
+
+    Served by ``GET /api/file/{vault_name}/xlsx/sheet``; the row numbers and
+    the ``data-cell`` references in ``html`` are the real A1 coordinates of the
+    sheet, whatever the window.
+    """
+
+    vault: str = Field(description="Vault name")
+    path: str = Field(description="Relative file path within the vault")
+    sheet: str = Field(description="Sheet name (as shown in the tab)")
+    offset: int = Field(description="0-based index of the first returned row")
+    limit: int = Field(description="Maximum number of rows returned (capped server-side)")
+    rows: int = Field(description="Rows actually returned in this window")
+    cols: int = Field(description="Columns of the rendered window")
+    total_rows: int = Field(description="Rows the sheet declares")
+    total_cols: int = Field(description="Columns the sheet declares")
+    max_rows: int = Field(description="Row cap of the renderer (500) — the coverage of this window")
+    max_cols: int = Field(description="Column cap of the renderer (40)")
+    truncated: bool = Field(
+        description="True when the sheet exceeds the 500x40 render caps"
+    )
+    has_more: bool = Field(description="True when rows remain after this window")
+    html: str = Field(description="Rendered HTML table for the window")
+
+
 class FileRawResponse(BaseModel):
     """Raw text content of a file."""