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/executions.css

@@ -8,15 +8,10 @@
    ===================================================================== */
 
 /* ---------------------------------------------------------------------
-   1. Modal sizing — base.css has .lt-modal-sm but no .lt-modal-lg, so the
-   large variant is scoped to this page's two modals to avoid colliding
-   with any other page that defines the same class.
+   1. Modal sizing — width comes from the shared `.lt-modal-lg` rule in
+   /assets/app.css; only the scroll behaviour of these two modals is
+   page-specific.
    --------------------------------------------------------------------- */
-#ex-detail-modal .lt-modal,
-#ex-compare-modal .lt-modal {
-  max-width: min(1100px, 94vw);
-  width: min(1100px, 94vw);
-}
 #ex-detail-modal .lt-modal-body,
 #ex-compare-modal .lt-modal-body {
   max-height: 72vh;