refactor(repo): conform structure to AGENTS.md (web->frontend, internal/api->cmd/webui/{api,handlers}, docs/README+CHANGELOGs, module .gitignores, drop stray library/ and committed debug bins)

docs/CHANGELOG.md

@@ -0,0 +1,10 @@
+# Changelog
+
+All non-WebUI user-visible changes. Newest version on top.
+
+## [Unreleased]
+
+### Changed
+
+- Repo structure conformed to `AGENTS.md`: `web/` renamed to `frontend/`; backend HTTP layer moved from `internal/api` to `cmd/webui/{api,handlers}` (`cmd/server` → `cmd/webui`); README/CHANGELOGs relocated under `docs/` (`README_zh.md` added as Chinese mirror); module-level `.gitignore`s added (`backend/`, `deploy/`); stray root `library/` removed (book files live in `deploy/api/storage/`); debug binaries untracked.
+- Docs: `docs/README.md` is now the English primary; previous Chinese README mirrored to `docs/README_zh.md`.