feat(ui): redo tx trend chart in dashboard sparkline style (rants 2026-08-24T10:51:57)

[argszero]

Summary

Redo the transactions-page "点数趋势" (points trend) chart in the style of the dashboard "本月点数变化" sparkline (host rants 2026-08-24T10:51:57.821209 + 2026-08-24T10:51:57.821928).

Fixes per rant 10:51:57.821209:

• Gradient area fill: 0.35 opacity at top fading to 0 at bottom (was 0.22 — rendered like a solid black block)
• Continuous smooth curve only — removed the hollow data-point circles that created visible gaps / jagged edges at inflection points
• Removed the dashed grid lines + heavy axis ticks (visual noise); minimal coordinates now: just top/bottom value labels
• Colors unified to the theme teal family: expense uses the teal accent (was danger red — the complaint), income ok-green, net accent-text, tokens warn
• Compact X-axis labels and legend

Fixes per rant 10:51:57.821928:

• viewBox height 190 → 85 (more than halved), tighter paddings, axis font 10px → 9px, legend 12px → 11px

Kept (per both rants):

• Metric switcher: 消费点数/收入点数/净变化/Token 用量, default 消费点数
• Hover tooltip + vertical guide line
• Elegant degradation: no dots regardless of point count

All changes are frontend-only (renderTxTrend + .tx-trend* CSS + cache-bust).

Related Issue

Host rants 2026-08-24T10:51:57.821209 (sparkline-style redo) and 2026-08-24T10:51:57.821928 (height halving). No GitHub issue.

Tests

• cargo test all pass (146/146)
• cargo fmt --check pass
• cargo clippy --all-targets -- -D warnings pass
• node --check on modified JS

Checklist

• Branch naming follows convention (feat/tx-trend-sparkline)
• Conventional Commits
• Single responsibility, minimal change