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.

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "melodymuse",
-  "version": "0.1.0",
+  "version": "0.2.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "melodymuse",
-      "version": "0.1.0",
+      "version": "0.2.0",
       "dependencies": {
         "jszip": "^3.10.1",
         "lucide-react": "^0.451.0",