fix(ui): export the transaction time column in the same 口径 as the cell

[argszero]

Summary

The transactions CSV export printed the time column in a different 口径 than the table cell it sits next to: exportTxCsv wrote the view row's raw t.time (the UTC string the API returns, which txsToView does not convert), while the cell renders the very same field through fmtPrecise() — i.e. local time. In a UTC+8 session one and the same row reads 23:04 in the table and 15:04 in the downloaded file (west of Greenwich the sign flips). A CSV whose header is localized (时间) and whose values disagree with the screen is a trap for anyone reconciling the file against the UI.

This is the same class of divergence the project already closed for the 点数 column in #196 (C2054/C2055), whose own comment states the rule: "the export covers the currently visible rows, so the column must use the same 口径 as the cell — otherwise the screen says -3.7 and the file says 3.7". Time is simply the column that was never brought along.

Provenance: the cell moved to precise local time in #139 (rant 2026-08-24T12:38:44); that commit updated the cell but not the other consumer of the same field (exportTxCsv, added earlier in #68).

Related Issue

N/A — no open issue. Background: #139 (cell 口径 changed to local), #196 (the same divergence fixed for the 点数 column).

Changes

• ui/js/app.js — the CSV 时间 column now uses fmtPrecise(t.time), the same helper that renders the cell, instead of a second copy of "timestamp to string" (one line + a comment stating why the two must stay同源).
• ui/README.md — the CSV convention section now records that every column shares the cell's 口径 (with time as the second instance after 点数).
• ui/index.html — cache-bust for the changed app.js (?v=20260914-6).
• No config / data-structure change, no new i18n key, no Rust change.

Tests

• cargo test — 235 passed (unchanged; the change is JS-only)
• cargo fmt --check — exit 0
• cargo clippy --all-targets -- -D warnings — clean
• node --check on all four ui/js/*.js
• jsdom probe (real ui/index.html + api.js/data.js/i18n.js/app.js, only fetch stubbed): boots three transactions, reads the rendered cell text and the bytes of the exported Blob (captured from URL.createObjectURL), and asserts that each CSV column equals its cell — time (the axis), plus type / pts / tokens as controls. Pre-change tree: 5/6, red on the time column only. Fixed tree: 6/6, exit 0, under both TZ=Asia/Shanghai and TZ=America/New_York.
  • The probe asserts a non-UTC process TZ first: under TZ=UTC the two 口径 coincide and the value divergence is invisible (it exists only in the format then), so a UTC run would be meaningless.
  • A/B legs: pre-change {time} · mirror-image repair (make the cell print UTC so the two "agree") {cell-local} — disjoint from the pre-change set, i.e. the instrument rejects consistency-by-breaking-the-cell · half fix (append ":00" and stay UTC) {time} (same assertion, different input — not mutually exclusive with the pre-change leg, recorded as such) · control-teeth leg (CSV type field printed raw) {type-control} · fixed tree {}.

Checklist

• Branch naming follows the convention (fix/…)
• Commit message uses Conventional Commits (fix(ui): …)
• Single responsibility, minimal change (3 files, +8/−3)