feat(sheets): CSV/XLSX export, multi-sheet XLSX import, export toolbar
CSV export: RFC 4180 compliant with UTF-8 BOM, delimiter options,
range export support. TSV variant.
XLSX export: maps cell styles back to Excel format (colors, fonts,
alignment, number formats). Preserves formulas. Cmd+Shift+E shortcut.
Multi-sheet XLSX import: imports ALL sheets from workbook (was first
sheet only), creates Yjs tabs for each, preserves sheet names.
Export toolbar: "Export CSV" and "Export XLSX" buttons in overflow menu.
70 new tests (33 csv-export, 27 xlsx-export, 10 xlsx-multi-sheet).