refactor: simplify architecture and harden lifecycle

[Awuqing]

Summary

• remove obsolete backend/frontend implementations and consolidate shared registries
• supervise application-owned background work and make shutdown/recovery deterministic
• harden Agent terminal-state reporting, startup reconciliation, scheduler rollback, and error logging
• centralize frontend route/menu metadata, split account security UI, and share SSE log parsing
• add ESLint/Prettier and strengthen Windows, docs, container, and release CI

Validation

• go test ./... -count=1
• go vet ./...
• go mod verify
• Linux/amd64 cross-build
• frontend lint, formatting, typecheck, 45 tests, production build, and production dependency audit
• English and Chinese documentation typecheck/build
• git diff --check

Docker was unavailable locally; the PR CI now performs Compose validation and a full container build.