fix: move sideways across the window cards of the session menu

[nextestudios]

Summary

In the session menu (overlay), moving sideways across the open-window cards could fail: left/right relied on the XY focus search alone, which can miss the neighbouring card (cards are scaled while focused and the grid is clipped by the scrolling panel).

• New MoveAcrossWindowCards in SessionMenuWindow: Left/Right on a window card picks the neighbour on the same row by position (the closest X on that side) and brings it into view. It runs after the existing "Left on the first column goes back to the side panel" rule and before the explicit "Right from the side panel enters the grid" rule from fix: restore DualShock navigation and release alpha 5 #130, so both keep working.
• The end of a row is an end: Right on the last card of a row is consumed so the search can't jump somewhere else.
• Doesn't change Up/Down, the picker/confirmation dialogs (which keep their own search root) or volume adjust mode.
• docs/TESTING.md: the sideways checks across one and several rows.

Validation

• UI behaviour, so no unit test; the manual checks are in docs/TESTING.md.
• Verified on a real PC with 3+ windows open (build from my fork), together with the other two fixes from this batch.
• App build and dotnet test are left to CI.

🤖 Generated with Claude Code

[pullfrog]

✅ No new issues found.

Reviewed changes

• Sideways card navigation: Left/Right on a focused window card selects the nearest card on the same row by position, brings it into view, and consumes Right at the row end while preserving the existing Left-to-sidebar behavior.
• Validation notes: Added paired changelog entries and manual test coverage for single- and multi-row card navigation.

  ^{｜ View workflow run ｜ Using GPT Luna ｜ 𝕏}