feat: volume-key page turning for e-ink + double-tap back

[mjhfunctionalashtanga]

What

Adds an opt-in Volume keys turn pages gesture (Settings → Gestures → Reader), aimed at e-ink devices:

• Volume-down pages the article forward ~90% of the viewport; volume-up pages back.
• At a scroll edge it rolls to the next / previous article.
• Double-tap volume-up = Back, dispatched through the normal back handler so it honors the existing back-action setting (close article / open drawer / navigate to parent).
• Also pages the article list.
• Yields to audio — while something is playing, the rocker still controls volume.
• Off by default.

Why

On e-ink readers (Boox, etc.) the hardware volume rocker is the natural way to turn pages hands-free without touching the screen — screen taps/swipes cause ghosting, and physical page keys are a defining e-ink affordance (à la EinkBro). This is a frequently-requested pattern for reading apps on those devices.

How it's built

• A small VolumeKeyPager singleton bridges MainActivity.dispatchKeyEvent (which consumes the volume keys only when the feature is active) to the reader/list scrollers registered from Compose.
• Discrete-press + repeat-count guards mean holding to page never misfires, and only volume-up double-taps trigger Back, so paging forward is always safe.
• Self-contained and independent of other features; ~230 lines, off by default.

Happy to adjust naming, the settings placement, the double-tap window, or gate it behind an "e-ink" section if you'd prefer.

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