Fix merged layout: right sidebar is a full-height sibling; plan graph renders

The weave had nested the right sidebar inside the transcript column
(below the header), so the phase chip sat above the panel instead of left
of it and the panel was bounded by the scroller area. The column is now
a true sibling of the middle column under flex h-dvh, per the UI stream's
original design; TreePanel's aside gets flex-1 min-h-0 so it shares the
column with the fleet panel instead of overflowing it. The plan-graph
viewport (ResizeObserver-measured) now has real size, so the mermaid
view draws.

Headless-verified at 1280x800: panel spans 0..800 top-to-bottom in both
modes, chip right edge < panel left edge in both modes, graph SVG = 9
boxes + 8 beziers, console clean.

components/tree-panel.tsx

@@ -236,7 +236,7 @@ export function TreePanel({
 		<aside
 			data-testid="tree-panel"
 			className={cn(
-				"sidebar-motion hidden shrink-0 overflow-hidden border-l border-border/60 bg-card/30 duration-200 xl:flex",
+				"sidebar-motion hidden min-h-0 flex-1 shrink-0 overflow-hidden border-l border-border/60 bg-card/30 duration-200 xl:flex",
 				PANEL_WIDTH[mode],
 			)}
 		>