feat(ui): custom SVG select arrow — appearance:none, themed, hover/focus like input (v1.19 C)

[argszero]

Summary

UI/UE 4th-round deepening — item C: native select styling (rant 2026-08-17T20:39:30, v1.19).

All native select elements now use a custom SVG dropdown arrow instead of the browser default, consistent with the dark/light themes:

• --select-arrow CSS variable: inline SVG data-URI (light arrow #aab6c8 for dark theme, dark arrow #55627a for light theme; spaces %20-encoded for safety)
• appearance:none + -webkit-appearance:none hides the native arrow; padding-right reserves arrow space (30px for .input, 24px for .th-filter, 22px for page-size)
• hover / focus border matches inputs (var(--accent), focus adds a 1px accent ring); disabled → opacity:.55 + not-allowed
• Covers every select source: static .input (market filters #mk-provider/#mk-sort, share-form 3-level cascade #sf-provider/plan/model, settings language/default-model), dynamic select.th-filter (table column filters), select[data-page-size] (pager rows-per-page)
• Preserved: option dark background (select.input option), input-error red border state
• Implementation detail: the select rules use background-color/background-image longhands (never the background shorthand) so the arrow image is never wiped; arrow rule declared after the shared .input shorthand so it wins by source order

Docs: docs/user-stories.md v1.19 (item C) + ui/README.md「select 美化约定」. Verified by node --check-style static smoke test (19 assertions: selector coverage of all static+dynamic selects, appearance:none, hover/focus, dual-theme arrows, no shorthand clobbering), cargo test, cargo fmt --check.

Rant (verbatim)

Rant: UI/UE 第四轮多角度深化 — 状态保持、信息密度、原生控件美化、帮助可达性（宿主 2026-08-17 第四轮指示：继续从多角度美化 UI、优化 UE）

C. 原生 select 美化

• 当前 select 是原生箭头，与深色主题不搭：自定义下拉箭头（CSS background SVG 箭头 + appearance:none + padding-right），hover/focus 边框同 input
• option 深色背景已适配（select.input option 已有），保持
• 覆盖市场筛选、共享表单、数据表 page-size、管理员表单等所有 select.input

Related

• Rant timestamp: 2026-08-17T20:39:30.507364 (project aitokenpool, status in_progress; A = feat(ui): URL hash routing — share/bookmark/refresh restore (v1.19 A) #55, B = feat(ui): transactions summary bar — income/expense/net tied to tab & column filters (v1.19 B) #56 merged)
• Branch: feat/ui-ue-round4-select-style (per rant: 分支 feat/ui-ue-round4; items D–G to follow)

Tests

• Static smoke test — 19/19 assertions pass (all select sources covered, appearance:none, themed arrows, no shorthand clobber)
• cargo test — 1 passed
• cargo fmt --check