fix(ui): transactions table time column shows precise local time (rant 2026-08-24T12:38:44)

[argszero]

Summary

Transactions table time column now shows precise local time (YYYY-MM-DD HH:MM:SS, e.g. 2026-08-24 12:36:12) instead of relative time ("3 小时前"), per rant 2026-08-24T12:38:44.

• New fmtPrecise(s) helper: parses backend UTC ISO / legacy YYYY-MM-DD HH:MM:SS (treated as UTC, same convention as timeAgo) and formats as local time to the second; non-standard strings (e.g. visitor mock MM-DD HH:mm) pass through unchanged
• timeCell(s, precise): with precise=true the exact time becomes the main cell text and the relative time ("3 小时前" / "昨天") moves to the title hover tooltip (swapped)
• Only the transactions table time column (TX_COLUMNS) uses precise mode; other timeCell users (market listing 上架时间, settings 最近使用) keep relative display unchanged

Cache-bust 20260824-6 → 20260824-7.

Tests

• cargo test 146/146, cargo fmt --check, cargo clippy green
• node --check + fmtPrecise smoke test: UTC→CST conversion, milliseconds strip, legacy format, mock passthrough, empty — all pass