feat(ui): toast queue — stack up to 3, independent fade in/out, keep grading styles (v1.19 D)

[argszero]

Summary

UI/UE 4th-round deepening — item D: toast queue stacking (rant 2026-08-17T20:39:30, v1.19).

Replaces the single overwriting #toast element with a queue container #toast-wrap (fixed bottom-center, vertical stack, gap:10px, pointer-events:none so it never blocks page clicks):

• toast(msg, type) now creates an independent element per call — consecutive operations no longer overwrite earlier messages
• Max 3 toasts (TOAST_MAX = 3): when exceeded, the oldest is removed immediately to make room
• Independent lifecycle: each toast fades out (.out + toast-out 0.2s animation) after 2.6s, then removes itself — no shared timer, no cross-talk; orphaned timers of evicted toasts are no-ops
• Grading styles preserved (.toast.success/.error/.info border + text colors unchanged); all 39 existing toast() call sites untouched
• .toast keeps pointer-events:auto (container is none) — future-proof for interactive toasts

Docs: docs/user-stories.md v1.19 (item D) + ui/README.md「toast 队列约定」. Verified by DOM-stub smoke test (21 assertions: queue append/order, 3-cap eviction, fade-out + removal, timer cleanup, container reusability, HTML/CSS structure, grading preserved), 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）

D. toast 队列堆叠

• 当前单 toast 会被连续操作覆盖：改为队列（最多同时 3 条，纵向堆叠右下角或底部居中），每条独立淡入淡出
• 已有分级样式保留（success/error/info）

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, C = feat(ui): custom SVG select arrow — appearance:none, themed, hover/focus like input (v1.19 C) #57 merged)
• Branch: feat/ui-ue-round4-toast-queue (per rant: 分支 feat/ui-ue-round4; items E–G to follow)

Tests

• DOM-stub smoke test — 21/21 assertions pass
• node --check ui/js/app.js
• cargo test — 1 passed
• cargo fmt --check