fix(ui): refine tx trend chart visual proportions (rant 2026-08-24T13:31:02)

[argszero]

Summary

Refine the transactions page "点数趋势" (points trend) chart visual proportions so it matches the delicate dashboard sparkline style (rant 2026-08-24T13:31:02: fonts too large, line too thick on v0.7.15).

Changes (CSS-only, ui/css/style.css + cache-bust bump):

• .trend-line stroke-width 1.8 → 1.4 (sparkline reference is 1.5; slightly finer per the rant's "或更细")
• .tx-trend-axis font 9px → 8px (axis ticks + time labels, all rendered via this class)
• .tx-trend-title12px → 11px, .tx-trend-legend11px → 10px, .tsw-btn (metric switcher) 12px → 11px with tighter padding 3px 12px → 2px 10px
• Tighter spacing: .tx-trend padding 10px 16px 8px → 8px 16px 6px, switch margin 6px 0 8px → 4px 0 6px, legend gap 12px → 10px / padding-top 4px → 2px, legend dot 7px → 6px (proportional to smaller text)

Unchanged (per rant requirement): gradient area, smooth curve, metric switcher (消费/收入/净变化/Token), hover tooltip — functionality preserved, visual proportions only.

Cache-bust 20260824-9 → 20260824-10.

Tests

• node --check OK (no JS changes)
• CSS diff reviewed against all 4 metric switch variants (shared classes, applies uniformly)
• No Rust changes — cargo test unaffected (146/146 baseline green on main)