feat(ui): micro-interactions — press feedback, stat lift, number bump, reduced-motion (v1.18 E)

[argszero]

Summary

Implements item E (动效与系统偏好 / Micro-interactions & system preference) of the UI/UE round-3 wave (rant 2026-08-17T18:06:09, branch feat/ui-ue-round3, version v1.18).

Three acceptance points:

1. Button press feedback — .btn:active:not(:disabled) { transform: scale(0.98) }, smooth via the existing transition: all .15s on .btn; disabled buttons excluded (pointer-events: none already blocks them).
2. Stat card hover micro-lift — .stat:hover lifts translateY(-2px) with border/shadow elevation, matching the existing .card:hover language (0.15–0.18s transitions).
3. Number/points change animation — new bump(el) helper (remove .bump → reflow → re-add, replaying @keyframes numJump: translateY(-3px) scale(1.02), 0.35s). Wired into all 4 balance-mutation points:
   • wallet top-up → #side-balance + #wallet-balance
   • chat consumption (deduct) → #side-balance
   • raise-request approval for the current user → #side-balance
   • operator top-up when topping up the current user (guarded by isMe so bumping another user doesn't animate an unchanged balance)

Plus prefers-reduced-motion: reduce — a global media block clamps animation-duration/transition-duration to 0.01ms, animation-iteration-count to 1, and scroll-behavior to auto: animations/transitions disabled, all functionality preserved.

Design notes

• Zero external dependencies (pure CSS + a tiny JS helper), consistent with the wave's constraints.
• #side-balance is an inline <b> — needs display:inline-block for transform to animate.
• Reduced-motion rule is global (*, *::before, *::after), so it covers the existing view/tbody fade-ins too.

Verification

• DOM-stub smoke test (13 assertions): top-up bumps both balance elements, consumption deduct bumps sidebar balance, raise approval bumps balance, bump replay works (remove→reflow→add) ✓
• node --check ✓ · cargo test ✓ · cargo fmt --check ✓

Rant (verbatim)

E. 动效细节与系统偏好

• 按钮按压反馈（:active 轻微下压 scale(0.98)）；统计卡 hover 微抬升
• 数字/点数变化时轻微跳动动画（如充值后余额）
• 尊重 prefers-reduced-motion: reduce：禁用过渡/动画（保留功能）

Docs

• docs/user-stories.md v1.18 entry extended with item E (remaining F–G noted)
• ui/README.md: new "动效与系统偏好约定" section