feat(ui): export transactions to CSV — current filtered rows, UTF-8 BOM, date-stamped filename (v1.20 E)

[argszero]

Summary

Item E of rant 2026-08-17T20:46:57 (UI/UE 第五轮多角度深化, v1.20): 交易记录导出 CSV — export transactions to CSV.

• 「导出 CSV」button (#tx-export-btn, right-aligned ghost button under the transaction tabs) in the Transactions view
• Exports the currently filtered rows — tab filter (全部/消费/收益) → filterRows(list, TX_COLUMNS, txTable.filters) — same data source as the table and summary bar (consistent with v1.19 B)
• CSV format: UTF-8 BOM (\uFEFF, no mojibake in Excel) + \r\n line endings; header 时间,类型,模型 / Key,Token 用量,点数,状态; type mapped to Chinese via TX_TYPE; points kept signed; RFC4180 double-quote escaping for fields containing ,/"/newlines
• Download: Blob(text/csv;charset=utf-8) → URL.createObjectURL → temp <a download> click → remove → revoke after 1s
• Filename: aitokenpool-transactions-YYYYMMDD.csv (local date)
• Empty result → info toast「没有可导出的交易记录」, nothing exported; success → toast「已导出 N 条交易记录」

Rant reference (verbatim)

E. 交易记录导出 CSV — Blob + a[download]，UTF-8 BOM，文件名 aitokenpool-transactions-YYYYMMDD.csv

Verification

• DOM-stub smoke test: 20/20 assertions (view + binding → Blob/anchor/download/filename → BOM bytes EF BB BF + header + full 13 rows → consume-tab subset (5) → pts column-filter subset (1) → empty no-export → HTML structure)
• node --check ui/js/app.js ✓, CSS braces balanced ✓
• cargo test ✓ (1 passed), cargo fmt --check ✓
• Docs: docs/user-stories.md v1.20 entry (E added), ui/README.md「交易记录导出 CSV 约定」section

Progress on rant 20:46:57: A (#63) + B (#65) + C (#66) + D (#67) + E (#68) merged; remaining F–G.