🐛 Remove dead css/ui.css <link> from frontend index template (#9840)

Fixes #9135.

The <link href="css/ui.css">  tag in
frontend/resources/templates/index.mustache references a CSS file that
the build pipeline never produces:

- compileStyles() in frontend/scripts/_helpers.js only writes main.css
  (always) and debug.css (dev-only) — there is no write to ui.css
- compileStorybookStyles() writes ds.css (design system), not ui.css
- No ui.scss source exists anywhere in frontend/resources/styles/

The reference was added in 45d04942c ("✨ Add example ui
storybook") but no corresponding build step was added to emit the file.

Result: every page load issues a request for /css/ui.css that nginx
returns as 404. In self-hosted Penpot deployments behind a reverse
proxy, the SPA's CSS init promise rejects on the 404, the React root
never mounts, and the user sees a black screen.

This patch removes the dead reference. If a future change actually
emits ui.css (or another distinct UI bundle), the <link> can be
re-added at that time.

Co-authored-by: Admin <admin@Admins-MacBook-Pro.local>

frontend/resources/templates/index.mustache

@@ -18,7 +18,6 @@
     <meta name="twitter:creator" content="@penpotapp">
     <meta name="theme-color" content="#FFFFFF" media="(prefers-color-scheme: light)">
     <link id="theme" href="css/main.css?version={{& version_tag}}" rel="stylesheet" type="text/css" />
-    <link href="css/ui.css?ts={{& version_tag}}" rel="stylesheet" type="text/css" />
     {{#isDebug}}
     <link href="css/debug.css?version={{& version_tag}}" rel="stylesheet" type="text/css" />
     {{/isDebug}}