fix: XLSX export producing corrupted files in Excel

[datlechin]

Summary

Fixes #464 — XLSX exports were corrupted when opened in Microsoft Excel (repair attempted but file appears empty).

Three issues in the custom XLSX writer:

• Missing cellStyles element in styles.xml — cellXfs entries referenced xfId="0" with no corresponding cellStyles definition. Excel couldn't resolve the style mapping, entered repair mode, and dropped all styled cells (including headers), making the file appear empty.
• No XML control character filtering — Illegal XML 1.0 control characters (0x00–0x08, 0x0B, 0x0C, 0x0E–0x1F) from binary/hex database columns produced malformed XML that Excel rejected entirely.
• Wrong ZIP version field — Local file headers declared version 2.0 (deflate) but used store method (0). Strict ZIP validators reject this mismatch. Corrected to version 1.0.

Test plan

• Export a MySQL table as XLSX, open in Microsoft Excel — verify data loads without repair prompt
• Export a table containing special characters (accented, CJK, emoji) — verify correct rendering
• Export a table with NULL values and binary data — verify no corruption
• Open exported file in Numbers.app — verify still works (regression check)