Add paste-your-songs manual import feature

Users can now paste a list of songs as text to create a playlist without
needing any service integration. Supports multiple formats: "Artist - Title",
"Title by Artist", "Artist: Title", and numbered lists. Includes a live
song count preview in the modal and free tier playlist limit enforcement.

frontend/src/pages/Discover.tsx

@@ -127,7 +127,7 @@ export default function Discover() {
           <textarea
             value={query}
             onChange={(e) => setQuery(e.target.value)}
-            placeholder='e.g., "Upbeat indie rock with jangly guitars" or "Dreamy synth-pop for late night drives"'
+            placeholder='e.g., "Upbeat indie rock with jangly guitars", "Dreamy synth-pop for late night drives", or just type artists/songs like "Radiohead, Tame Impala"'
             rows={3}
             className="w-full px-4 py-3 bg-cream/50 border border-purple-100 rounded-xl text-charcoal placeholder:text-charcoal-muted/40 focus:outline-none focus:ring-2 focus:ring-purple/30 focus:border-purple transition-colors text-sm resize-none"
           />