docs(ui): describe the login page the two-column redesign left behind

[argszero]

Summary

ui/README.md still documented the login page the way it looked before the two-column redesign.

Two sections are affected, for the same reason: e33bf2e (#153, "redesign login page as two-column layout + rework sidebar", 2026-09-11) replaced the single centered card, deleted the CSS those lines describe, and the README was never updated.

• ## 品牌与登录页氛围约定（v1.20 … G） — three of its bullets named selectors that no longer exist:
  • .login-view::before (44px grid + radial mask)
  • .login-view::after (accent glow) + the login-float keyframes
  • the .logo accent ring and the .logo::after inner highlight
• ## 登录页约定（v1.19 … G） — its "visual polish" bullet named .login-card .logo (52px glow) and .login-brand h1 (22px), both gone as well.

This file was the only carrier left. git grep -nE "login-float|login-view::before|login-view::after|logo::after" -- ui/index.html ui/css/style.css ui/js/ returns nothing, and rgba(78,205,196 survives in exactly three places — all of them in this README.

What changed

Both sections are corrected in place, naming the redesign that invalidated them (the idiom this file already uses — see the endpoint-card section's "（v1.22 全站 UI 重设计对齐原型…）") and stating what the login page actually is today: .login-brand (.eyebrow / h1.lb-headline / .lb-points / .gw) alongside the frosted .login-theme toggle, with the logo rendered by .logo-tile.

Claims that are still true are kept: the favicon bullet (inline SVG data URI, verified in ui/index.html), the focus ring .login-form .input:focus, and the global prefers-reduced-motion rule.

Tests

• cargo test — 413 passed, 0 failed
• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean

Docs only: no production code, no i18n key, no cache-bust token touched.

Deliberately not in this PR

The "记住我" bullet of ## 登录页约定 also names a demo-hint element that no longer exists in ui/index.html. That one was removed by a different change — 35291b3 (v0.6.0, demo-seed removal), not the redesign — so it gets its own PR instead of being mixed in here.

[argszero]

✅ Self-review (Committer; allow_self_merge: true).

Docs-only change, ui/README.md +4/−5. Verified independently of the text's own claims:

• every selector the new text names exists today: lb-headline, logo-tile, login-theme, lb-points, login-brand, eyebrow, gw;
• every selector the old text named is absent outside ui/README.md: login-float, login-view::before, login-view::after, logo::after, and rgba(78,205,196;
• the two claims deliberately kept — the favicon inline SVG data URI and the global prefers-reduced-motion rule — are still true in ui/index.html / ui/css/style.css;
• CI green on this head: test / fmt / clippy 2m59s, msrv 25s.

Merging.