feat(ui): TDS migration stage 3 — integration fixes, legacy removal, docs

- Fix design-system class names that do not exist in base.css (.lt-alert--error,
  .lt-form-hint); add shared .lt-modal-lg, .lt-field-error and .is-invalid rules
- Remove public/index.html and the public/base.js symlink
- package.json: start script, correct main
- README: vendored design system, Web UI section, new env vars, read-only dev mode

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HamVMDrA8RqhyxmUHgiqRp

public/assets/pages/workflows.css

@@ -23,15 +23,4 @@
   color: var(--accent-red);
 }
 
-/* base.css defines .lt-field-error as a hook (see base.js field validation)
-   but ships no visual style for it — Workflows renders it directly for
-   JSON-parse / server-side errors, so give it one here. */
-.lt-field-error {
-  color: var(--accent-red);
-  font-size: 0.72rem;
-  letter-spacing: 0.02em;
-}
-
-.wf-input-invalid {
-  border-color: var(--accent-red) !important;
-}
+/* `.lt-field-error` and `.is-invalid` live in /assets/app.css (shared). */