Add aria-valuetext to the seeking, volume and balance sliders

[captbaritone]

Stacked on #1369 (keyboard seeking), since both touch Position.tsx.

The seeking, volume and balance sliders are native range inputs, so screen readers announce their raw value: the seeking bar reads as "26", which means nothing. This adds aria-valuetext:

Slider	Example
Seeking Bar	"0:32 of 2:00". While dragging, it shows the scrub position, matching what the marquee shows.
Volume Bar	"75%"
Balance	"Center", "20% Left", "20% Right"

The seeking bar's text uses timeElapsed directly rather than reconstructing it from the slider's percentage. That avoids floating-point error showing "0:31" for 32 seconds.

The EQ band sliders get equivalent value text ("+6.0 dB") in #1370.

Testing

• New Position.test.tsx cases cover the text both idle ("0:30 of 2:00") and while scrubbing ("1:00 of 2:00").
• New Balance.test.ts covers balanceValueText.
• tsc, eslint, prettier --check and vitest run pass locally.

[github-actions]

Size Change: +210 B (+0.04%)

Total Size: 490 kB

📦 View Changed

Filename	Size	Change
packages/webamp/built/webamp.bundle.min.js	298 kB	+105 B (+0.04%)
packages/webamp/built/webamp.lazy-bundle.min.js	192 kB	+105 B (+0.05%)

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