Nod composer: show every line while typing, up to five, then scroll

[scgopi]

In Nod's chat pane, the draft box grew to two lines but the second line could be out of view, so you couldn't see what you were typing.

Change	Detail
Lines stay visible	After every resize the box shows all of its text when it fits (scrolls back to the top) and the caret's line when it doesn't. AppKit scrolled to the caret while the text outgrew the box, and that offset survived SwiftUI's resize
Grows to five lines	Height = used text height, at least one line, at most five (was a 140pt cap, about nine lines)
Then scrolls	Overlay scroller that autohides; the caret's line is kept in view while typing
Text view sizing	Explicit min/max size and an unbounded container height, so it grows reliably

Render: NodChatRenderTests.theComposerGrowsToFiveLines — two lines both visible; seven lines show 3–7 with the caret on the last.

Evidence

RED: NodComposerHeightTests.aBoxWhoseTextFitsIsNeverLeftScrolled with the guard removed -> fails, visible(box).minY is one line down; aLongDraftShowsFiveLinesAndScrolls on main's composer -> fails, box height is not 5 lines and there is no scroller
GREEN: xcodebuild test -only-testing NodComposerHeightTests -> 5 tests passed, including typing into the pane's own NodComposerView with its store
REGRESSION: full xcodebuild test -> 2234 tests, only MessageDeliveryTests.aSessionWhoseTaskEndedIsNeverTypedInto fails, identically on main on this Mac; cli and daemon builds exit 0; make check 0 errors; swift-portable exit 0