feat: show equalizer bars while loading

[emakko]

What

Replaces the plain grey loading text with four bouncing green equalizer bars above the same text.

• New src/components/Loading.tsx: the bars plus the message, with role="status". The bars are aria-hidden, so screen readers hear the same text as before.
• Used on all four loading states: "Connecting to Spotify…" (App.tsx), "Loading your playlists…" (PlaylistPicker.tsx), and "Loading songs…" (SwipeScreen.tsx, DedupeScreen.tsx).
• styles.css: pure CSS. Each bar scales between 30% and 100% of its height, and the bars have different durations and delays so they don't move in lockstep. Under prefers-reduced-motion they stand still at different heights.

Notes for review

• This touches the same PlaylistPicker.tsx as feat: slide the highlight when switching modes #7, but on different lines, so the two should merge in either order.

Testing

• npm test: 129 passed. npm run build: succeeds.
• Not checked in a browser. The preview setup wasn't available this time. Please take a look at app start or when opening a playlist before merging.

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