docs: backfill CHANGELOG and README for 1.5.4–1.5.6

[Cmochance]

Summary

Backfilling docs that lagged behind shipped features. Pure docs PR, no code changes.

• CHANGELOG.md: adds entries for 1.5.4 / 1.5.5 / 1.5.6. The file was last updated for 1.5.3 — every release since then shipped without a changelog entry.
• README.md / README.zh-CN.md: updates "当前功能" so the 账号页 mentions click-to-cancel during login, and 设置页 mentions the new "Codex CLI 路径" row. Both files are kept in sync.

Context

Per the project rule that user-visible behavior in a code PR should ship docs in the same PR (now codified in memory), PR #27's codex-CLI-path fallback + cancellable login should have updated these. This PR closes that gap.

Test plan

• Skim CHANGELOG entries for accuracy against the actual commits in v1.5.4 (feat(quota): cherry-pick ChatGPT-API direct quota refresh onto 1.5.x #15–fix(login): resolve real codex CLI from live home, not sandbox #18), v1.5.5 (feat(plan): freshen plan_name with API plan_type + JWT fallback (Phase A) #19–fix(switch): unstick "switch already in progress" toast (stale lock + double-click) #24), v1.5.6 (fix(plan): trust API plan_type on downgrade; clear stale quota when API empty #25, feat(login): surface and recover from missing codex CLI #27).
• Skim README "当前功能" — confirm the 账号页 + 设置页 lines match what the app actually does today.

🤖 Generated with Claude Code

[Cmochance]

剩余在你侧

1. Skim CHANGELOG — 1.5.4 / 1.5.5 / 1.5.6 三段是基于 commit 信息回溯写的，1.5.4 / 1.5.5 不是我当时做的，可能漏 / 错条目。读一遍补漏。
2. Skim README — "当前功能" 段我只加了 codex CLI 路径 + 登录可取消两条，没动其它。确认读起来连贯。
3. CI 全绿后 squash-merge — 纯文档 PR，无 reviewer 阻塞。
4. 不需要发新 release — 1.5.6 release notes 用的是 GitHub auto-generated（含本 PR 的 commit）；如果想把 CHANGELOG 内容反向编辑进 release body，单独操作即可。

跟 PR #26 关系

PR #26（baseurl button）独立分支，仍 OPEN。本 PR 没动 PR #26 的范围；如果 PR #26 后续合入会变成 1.5.7，届时记得把 1.5.7 的 CHANGELOG 条目也补上（按本 PR 同样格式）。