fix: use relative asset paths with always-injected <base> tag

Switch Vite base back to './' (relative paths) and always inject
<base href="${BASE_PATH}"> in the entrypoint, even when BASE_PATH=/.

This fixes asset loading for both deployment modes:
- Single-instance: <base href="/"> resolves ./assets/x.js to /assets/x.js
- SaaS tenant: <base href="/t/slug/"> resolves to /t/slug/assets/x.js

Previously base:'/' produced absolute /assets/ paths that the <base>
tag couldn't redirect, breaking SaaS tenants. And base:'./' without
<base> broke deep URLs in single-instance mode. Always injecting the
tag makes relative paths work universally.

The patched server-ui-entrypoint.sh in cameleer-saas (which rewrote
absolute href/src attributes via sed) is no longer needed and can be
removed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/vite.config.ts

@@ -27,7 +27,7 @@ export default defineConfig({
   optimizeDeps: {
     include: ['swagger-ui-dist/swagger-ui-bundle'],
   },
-  base: '/',
+  base: './',
   build: {
     outDir: 'dist',
   },