feat: harden shared platform contracts

webui/scripts/test-dialog-focus-structure.mjs

@@ -10,6 +10,8 @@ const stackSource = readFileSync("src/components/dialogStack.ts", "utf8");
 assert(source.includes("ref={panelRef}"), "Dialog wires its panel ref to the focus boundary");
 assert(source.includes("tabIndex={-1}"), "Dialog exposes a programmatically focusable fallback");
 assert(source.includes("return registerDialog({"), "Dialog registers with the shared modal stack");
+assert(source.includes("createPortal(dialog, document.body)"), "Dialog supports opt-in body portals without replacing the shared stack");
+assert(source.includes("panelStyle"), "Dialog accepts bounded custom panel positioning");
 assert(!source.includes('window.addEventListener("keydown"'), "Dialog instances do not install competing keyboard listeners");
 assert(stackSource.includes('window.addEventListener("keydown", handleWindowKeyDown)'), "the modal stack owns one keyboard listener");
 assert(stackSource.includes("handleTopDialogKeyDown(event, currentActiveElement())"), "the keyboard listener delegates only to the topmost dialog");