Fix: raise swipe velocity threshold to prevent accidental navigation in message viewer

[digitalby]

Fixes #10747

Summary

A single constant change in MessageViewContainerFragment raises the velocity threshold for
the swipe-between-messages gesture, preventing fast but unintentional gestures from navigating
to an adjacent email and marking it as read.

The distance threshold (VIEW_PAGER_SWIPE_THRESHOLD = 0.4f) is unchanged.

Notes on the constant value

3.0f is not a final answer. The absolute dp/s value depends on the device. However, this makes accidental velocity-based swipes much harder to trigger. It does not:

• Prevent a deliberate but regretted swipe from marking an email as read
• Add a delay before mark-as-read fires
• Add any visible indicator of read state in the message viewer

Those are separate concerns that I've raised an issue about.

Testing

• Manual: open message list, make a fast short flick left/right → should not navigate
• Manual: make a deliberate fast swipe left/right → should still navigate normally
• Manual: make a slow deliberate drag > 40% width → should still navigate normally
• Existing tests: ./gradlew :legacy:ui:legacy:test

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