feat(ui): trend chart polish + metric switcher (rant 2026-08-23T16:17:18)

[argszero]

Summary

Polish the transaction-page trend chart and add a metric switcher (host rant 2026-08-23T16:17:18):

Requirement 1 — chart aesthetics:

• Smooth Catmull-Rom → cubic Bézier line (replaces angular polyline)
• Optional gradient area fill under the line (per-metric translucent fade)
• Theme-consistent per-metric colors (Spend=danger, Income=ok, Net=accent, Tokens=warn) — adapts to dark/light themes via CSS variables
• Dashed grid + cleaner axis labels
• Hover tooltip with precise value + vertical guide line (replaces native <title>)
• Adaptive y-axis scale: nice steps (1/2/5×10^n); Net uses a symmetric ±max axis so the zero line stays visible

Requirement 2 — selectable metric:

• Metric switcher buttons: 消费点数 Spend / 收入点数 Income / 净变化 Net / Token 用量 Tokens
• Default shows 消费点数 (Spend); switching re-renders instantly (no extra API call — /api/transactions/trend already returns all metrics)
• Legend shows the selected metric's period total

Related Issue

Host rant 2026-08-23T16:17:18 (trend chart polish + metric switcher) — no GitHub issue.

Changes

• ui/js/app.js: renderTxTrend rewrite (switcher, smooth path, gradient area, adaptive scale, tooltip context) + delegated click/mousemove/mouseleave bindings + txTrendMetric state
• ui/css/style.css: switcher, chart wrapper, guide line, tooltip, gradient stops, per-metric colors
• ui/js/i18n.js: zh/en keys tx.trend.metric.*
• ui/index.html: cache-bust 20260823-3 → 20260824-1

Tests

• cargo test all pass (146/146)
• cargo fmt --check pass
• cargo clippy --all-targets -- -D warnings pass (no new warnings)
• node --check on app.js / i18n.js

Checklist

• Branch naming follows convention (feat/trend-chart-polish)
• Commit message uses Conventional Commits
• Single responsibility, minimal change