Make the seeking bar work from the keyboard

[captbaritone]

The seeking bar (#position, an <input type="range">) can be focused, but only seeks in onPointerUp. Arrow keys fire onInput, which only updates the scrub preview. The clock display moves, but playback never seeks, and the slider stays stuck in "scrubbing" mode until the next pointer interaction.

Changes

• Arrow keys seek ±5 seconds (Right/Up forward, Left/Down back), using the existing seekForward/seekBackward action creators that the arrow-key hotkeys use. I didn't keep the native 1% step: the displayed position uses Math.floor(timeElapsed), so on tracks shorter than 100s a 1% step rounds back to the same value, and a paused track would never get past the start.
• Other non-pointer input seeks immediately. That's Home, End, PageUp/PageDown, and screen-reader increment/decrement actions.
• Pointer dragging is unchanged. It still previews while dragging and seeks on release.

The global hotkeys already ignore events from <input>s, so with enableHotkeys on there's no double seek.

Testing

• New Position.test.tsx (thirty seconds into a two-minute track):
  • ArrowRight seeks to 35s and ArrowLeft to 25s, and both prevent the native step.
  • Other keys are left alone.
  • Input with no pointer down seeks immediately.
  • A pointer drag only seeks on pointerup.
• The three keyboard tests fail against the old implementation. The pointer-drag test passes against both, confirming that behaviour is unchanged.
• tsc, eslint, prettier --check and vitest run pass locally.

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[github-actions]

Size Change: +406 B (+0.08%)

Total Size: 491 kB

📦 View Changed

Filename	Size	Change
packages/webamp/built/webamp.bundle.min.js	298 kB	+186 B (+0.06%)
packages/webamp/built/webamp.lazy-bundle.min.js	192 kB	+220 B (+0.11%)

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