Make the equalizer's +12/0/-12 dB buttons keyboard operable

[captbaritone]

Builds on #1365 (merged), which makes WinampButtons with an onClick into focusable role="button" elements.

The equalizer's "+12db", "+0db" and "-12db" labels are part of the background sprite. Clicking one sets every band to that level, through transparent divs with an onClick. Those divs weren't focusable, had no role, and had no name.

This renders them as WinampButtons named "Set all bands to +12 dB", "Set all bands to 0 dB" and "Set all bands to -12 dB".

• Rendering is unchanged. They're still the same transparent divs with the same ids. While pressed, WinampButton adds a winamp-active class, but no skin CSS targets it for these ids.
• Tab order: On, Auto, Presets, Preamp, +12, 0, −12, then the bands, which follows the window's layout.

Testing

tsc, eslint, prettier --check and vitest run pass locally.

[github-actions]

Size Change: +80 B (+0.02%)

Total Size: 490 kB

📦 View Changed

Filename	Size	Change
packages/webamp/built/webamp.bundle.min.js	298 kB	+38 B (+0.01%)
packages/webamp/built/webamp.lazy-bundle.min.js	192 kB	+42 B (+0.02%)

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