[P1] Reduce ReaderViewport interaction-mode confusion

[nkanf-dev]

Summary

Simplify or clarify the pointer vs grab interaction model in the reader so the viewport behaves closer to user expectations.

Why

The current mode split creates a learning cost and can make basic scrolling feel broken when users do not understand which mode they are in.

Current evidence

• src/components/reader/ReaderViewport.tsx changes overflow behavior based on interaction mode.
• The current UI requires users to understand two modes before basic navigation feels natural.

Scope

• Re-evaluate whether both modes are necessary.
• If both remain, improve affordances and defaults so the active mode is obvious.
• Reduce surprising wheel/scroll behavior.
• Add tests for expected viewport interactions.

Acceptance criteria

• Basic reading and scrolling work intuitively for first-time users.
• The active interaction mode is obvious or no longer necessary.
• Mode changes do not create inconsistent scroll behavior.

Out of scope

• Full gesture redesign for touch devices.