Fix CSS: add postcss config, safelist stagger-N, drop dead rules

The page was loading with effectively no Tailwind utility classes
because there was no postcss.config.js — Vite didn't know to run
Tailwind's PostCSS plugin. The preflight and @apply references in
src/index.css were partially working (because some Tailwind processing
was happening implicitly), but the JIT content scanner wasn't running,
so all utility classes (flex, grid, p-*, text-fg, bg-bg, …) were
missing from the build. The page rendered as unstyled HTML.

postcss.config.js — newly added. Tells Vite to run tailwindcss +
autoprefixer. Written in plain UTF-8 (the earlier draft saved as
UTF-16 with BOM, which Node couldn't parse, hence the 'Syntax Error'
in the file viewer).

tailwind.config.js — added a safelist for the dynamic
`stagger-${index}` classes used in ResultCard.tsx. The JIT
content scanner can't resolve a template literal, so without the
safelist the four matching rules (stagger-0 … stagger-4) got purged
from the build even though they're used at runtime. Adding them to
the safelist keeps them alive.

src/index.css — dropped three dead rules (card-hover, btn-primary,
gradient-bg-soft) that the modern UI redesign made unused. They
were in the file but never referenced by any .tsx, so the
content-aware purger was stripping them anyway; explicit removal
keeps the source honest.

postcss.config.js

@@ -0,0 +1,11 @@
+// PostCSS config for Vite. Tells Vite to run Tailwind's JIT compiler and
+// Autoprefixer on every CSS file it processes. Without this, Vite has no
+// way of knowing which PostCSS plugins to apply, and the @tailwind
+// directives in src/index.css would either be ignored or only partially
+// processed (preflight + @apply but no utility classes).
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+};