build: remove Tailwind, which was never actually running

Tailwind was configured but had no effect. There is no postcss config anywhere
in the repo, so the three `@tailwind` directives in the backoffice stylesheet
were never expanded — they passed through verbatim into the production CSS
bundle as invalid at-rules for browsers to discard. The portal's copy could not
even do that: nothing imports `apps/portal/src/styles.css`.

The utility classes visible in the built CSS come from the vendored
@tria-plc/iamui package, which ships its own pre-compiled stylesheet and
declares only React as a peer dependency. Removing Tailwind here does not
affect it.

`tailwind-merge` goes too — no file in apps/ or libs/ imports it. postcss and
autoprefixer stay; Vite uses them independently.

Also fixes a bug this uncovered. The orphaned portal stylesheet was not dead
code — it contained scrollbar theming the portal never received, so it has been
rendering raw OS scrollbars while the backoffice got themed ones. Those rules
move to portal.css, which actually loads. Deleting the file without reading it
would have silently kept that gap.

No visual change: all 8 baselines pass unmodified.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/backoffice/src/styles.css

@@ -1,7 +1,4 @@
 @import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap');
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
 
 *, *::before, *::after { box-sizing: border-box; }