feat: completude de l'editeur Excel - mise en forme, sortie, concurrence, cache, outils IA #156

L'editeur lisait les styles mais ne les ecrivait pas, exportait la feuille
entiere et laissait le dernier-ecrivain gagner entre processus.

- A8 : bouton Mise en forme (gras/italique/souligne, alignements, couleurs via
  selecteur natif, formats de nombre, fusion, volets figes, largeur/hauteur) et
  nouvelle route PUT .../xlsx/style (verrou, backup, swap atomique, garde de
  perte, If-Match) ; A9 : decision "pas de moteur de formule" annoncee dans
  l'UI ; A10 : undo/redo unifie, piles par fichier conservees au re-rendu
- A11 : export de la selection + Markdown/HTML/impression et recherche sur
  toutes les feuilles ; A12 : concurrence optimiste (If-Match -> 409 reparable,
  retry qui relit) ; A13 : cache des metadonnees par (chemin, mtime, taille)
- A14 : outils IA .xlsm/.csv + search_workbook, analyze_range,
  edit_xlsx_structure
- tests : test_xlsx_styles.py (17), test_spreadsheet_tools.py (34),
  TestOptimisticConcurrency/TestMetaCache, JSDOM xlsx-viewer 107/107

frontend/locales/en.json

@@ -1848,13 +1848,59 @@
   "xlsx.load_error": "Could not load the remaining rows",
   "xlsx.formula_bar_placeholder": "Active cell content",
   "xlsx.active_cell": "Cell",
-  "xlsx.find_placeholder": "Search in the sheet…",
+  "xlsx.name_box_label": "Name Box — jump to a cell or a range",
+  "xlsx.name_box_hint": "Type a reference (B12, A1:B3) then Enter to jump there",
+  "xlsx.name_box_invalid": "Unknown reference: {ref}",
+  "xlsx.copy": "Copy",
+  "xlsx.cut": "Cut",
+  "xlsx.paste": "Paste",
+  "xlsx.paste_empty": "Clipboard empty or unreadable",
+  "xlsx.paste_out_of_grid": "Part of the pasted block is outside the displayed grid and was skipped",
+  "xlsx.fn_suggestions": "SUM;AVERAGE;COUNT;COUNTA;MIN;MAX;IF;VLOOKUP;XLOOKUP;SUMIF;SUMIFS;COUNTIF;CONCAT;LEFT;RIGHT;MID;TODAY;NOW;ROUND;ABS;SQRT",
+  "xlsx.formula_stored_as_text": "{n} value(s) starting with “=” or “@” saved as text — turn on the f(x) button to write real formulas",
+  "xlsx.find_placeholder": "Search in the workbook…",
   "xlsx.find_prev": "Previous",
   "xlsx.find_next": "Next",
   "xlsx.find_case": "Match case",
   "xlsx.find_no_match": "No match",
   "xlsx.find_count": "{index}/{count}",
-  "xlsx.csv_export": "Export the sheet as CSV",
+  "xlsx.find_count_sheets": "{index}/{count} · {sheets} sheets",
+  "xlsx.csv_export": "Export the sheet or the selection as CSV",
+  "xlsx.export_btn": "Export",
+  "xlsx.export_title": "Export the selection or the sheet",
+  "xlsx.export_md": "Export as Markdown",
+  "xlsx.export_html": "Export as HTML",
+  "xlsx.print": "Print",
+  "xlsx.export_scope_sheet": "the sheet",
+  "xlsx.export_scope_selection": "the selection",
+  "xlsx.export_done": "Exported {scope}",
+  "xlsx.format_btn": "Formatting",
+  "xlsx.format_bold": "Bold",
+  "xlsx.format_italic": "Italic",
+  "xlsx.format_underline": "Underline",
+  "xlsx.format_clear": "Clear formatting",
+  "xlsx.format_align_left": "Align left",
+  "xlsx.format_align_center": "Center",
+  "xlsx.format_align_right": "Align right",
+  "xlsx.format_font_color": "Text colour",
+  "xlsx.format_fill_color": "Fill colour",
+  "xlsx.format_num_general": "General format",
+  "xlsx.format_num_number": "Number (2 decimals)",
+  "xlsx.format_num_percent": "Percentage",
+  "xlsx.format_num_currency": "Currency (€)",
+  "xlsx.format_num_date": "Date (DD/MM/YYYY)",
+  "xlsx.format_num_text": "Text",
+  "xlsx.format_merge": "Merge cells",
+  "xlsx.format_unmerge": "Unmerge cells",
+  "xlsx.format_merge_needs_range": "Select at least two cells to merge",
+  "xlsx.format_freeze": "Freeze panes here",
+  "xlsx.format_unfreeze": "Unfreeze panes",
+  "xlsx.format_col_width": "Column width…",
+  "xlsx.format_col_prompt": "Width of column {col} (0 to 255)",
+  "xlsx.format_row_height": "Row height…",
+  "xlsx.format_row_prompt": "Height of row {row} (0 to 409)",
+  "xlsx.format_applied": "Formatting saved",
+  "xlsx.format_error": "Formatting failed",
   "xlsx.sort_asc": "Sort column A→Z",
   "xlsx.sort_desc": "Sort column Z→A",
   "xlsx.sort_applied": "Sort applied on {col} — display only, the workbook is unchanged",
@@ -1905,6 +1951,7 @@
   "xlsx.dashboard_nr_ref": "Reference",
   "xlsx.dashboard_hint": "Select a range or open the AI assistant to analyse this data.",
   "xlsx.feature_cached_values": "cached values",
+  "xlsx.feature_cached_values_unverified": "cached values could not be verified (workbook too large)",
   "xlsx.feature_slicers": "slicers and timelines",
   "xlsx.feature_form_controls": "form controls",
   "xlsx.feature_connections": "connections and queries",
@@ -2080,6 +2127,9 @@
   "ai.step.xlsx_read": "Workbook read: {value}",
   "ai.step.xlsx_update": "Cells edited: {value}",
   "ai.step.xlsx_append": "Rows appended: {value}",
+  "ai.step.xlsx_search": "Workbook searched: {value}",
+  "ai.step.xlsx_analyze": "Range analysed: {value}",
+  "ai.step.xlsx_structure": "Structure changed: {value}",
   "ai.step.docx_create": "Word document proposed: {value}",
   "ai.step.csv_create": "CSV file proposed: {value}",
   "ai.step.pdf_create": "PDF document proposed: {value}",