fix: keep full-height frames off the primary buffer during the alt-screen hop

[hbrooks]

Important

Prevents full-height UI frames from rendering during the alternate screen transition in SessionsApp, which would cause them to paint on the primary buffer and shift the shell's scrollback history upward.

• During the alt-screen hop (when transitioning to alternate buffer mode), fullFrameHopGap returns early with an empty box instead of rendering the navigation picker or other full-height screens.
• Frames that render before the transition settles land on the primary buffer instead of the alternate buffer, which inadvertently scrolls the shell's history and chat transcript up into scrollback — the exact space the chat's scrollback view is meant to preserve.

^{This description was created by for ab0311f. It will automatically update as commits are pushed.}

[ellipsis-dev]

Caution

Changes requested ❌ — 1 issue

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[ellipsis-dev]

The hop gap is missing on the other primary→alt transition: ctrl+r's transcript browser still paints a full-height frame on the primary buffer. ConnectApp calls useAltScreen(windowed) at src/ui/ConnectApp.tsx:315 and throws the settled flag away, so the frame where windowed flips true commits with minHeight={rows - bottomSlack} (src/ui/ConnectApp.tsx:1526) and the <Static> transcript withheld — before the 1049h write.

In a chat whose transcript reaches the bottom of the terminal, pressing ctrl+r paints a screenful-tall frame at the cursor, scrolling the flushed transcript rows up; 1049h then saves the already-damaged primary buffer, so esc restores the shifted history. Same failure this PR fixes for the picker, reached by a different key.