fix(app): hide the closed side region over the side terminal

[Hona]

Issue for this PR

No linked issue.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

With the terminal docked on the side, a straight light line can sit on the terminal panel's top edge and stick out past its rounded corners.

• Cause: when the side region (Review, Browser, …) closes while the side terminal is open, it stays mounted at zero height. The close animation hides it with visibility: hidden, but only while data-opened="false" is set. Anything that recomputes sideMotion without a region change, such as turning the terminal off and on, clears data-opened. The zero-height #review-panel then shows again. A zero-height box cannot round its corners, so its --v2-elevation-raised ring paints as a flat line across the full column width, on top of the terminal panel's rounded top edge.
• Fix: keep the region hidden while it is closed, except during its own close animation.

<div
  data-slot="session-side-region-presence"
  data-opened={sideMotion().animateRegion ? sideMotion().region : undefined}
  class="absolute inset-0"
  classList={{ invisible: !screen.side.region.open() && !sideMotion().animateRegion }}
>

The region still collapses with its animation, reopens normally, and stays mounted while the side terminal is open.

How did you verify your code works?

• Desktop dev build, dark theme: open Review, open the side terminal (ctrl+backquote), close Review, then turn the terminal off and on.
• Before: the closed region is visible, 0px tall, at the terminal's top edge. After: the same element is visibility: hidden, and reopening Review restores it to its full height.

Screenshots / recordings

Top of the side terminal panel, dark theme, 2x zoom:

Before	After

Top-left and top-right corners, 10x zoom, dark range brightened:

Before	After

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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