fix: keep the opening chat text out of the URL

A long first message in ?prompt= exceeds Node's header limit. The chat GET returns 431 and Electron commits an empty window. Stash the text in sessionStorage and open /chat/<id>.

components/opening-prompt.ts

@@ -0,0 +1,41 @@
+/**
+ * The opening message rides beside the navigation, never in the URL.
+ *
+ * Home and the new-session draft used to push `/chat/<id>?prompt=<text>`.
+ * Node's default max HTTP header size is 16KB. A long paste (a syllabus)
+ * makes that GET return 431, and Electron commits an empty document: the
+ * window stays the native background and looks frozen. The chat record
+ * already exists, with an empty transcript, because POST /api/chats
+ * succeeded first.
+ *
+ * sessionStorage holds the trimmed text under `lh.openingPrompt.<id>`.
+ * The push target stays `/chat/<id>`. The chat page still honors a
+ * `?prompt=` search param for an old link; when that param is absent it
+ * reads this stash and sends it once. The stash survives until that send
+ * resolves, so a reload before the POST lands can retry. Absent storage
+ * (node tests, a thrown access) is a no-op — the short path still returns.
+ */
+
+const keyFor = (id: string) => `lh.openingPrompt.${id}`;
+
+function store(): Storage | null {
+	try {
+		if (typeof sessionStorage === "undefined" || sessionStorage == null) return null;
+		return sessionStorage;
+	} catch {
+		return null;
+	}
+}
+
+export function stashOpeningPrompt(id: string, text: string): void {
+	store()?.setItem(keyFor(id), text);
+}
+
+export function readOpeningPrompt(id: string): string | undefined {
+	const value = store()?.getItem(keyFor(id));
+	return value && value.length > 0 ? value : undefined;
+}
+
+export function clearOpeningPrompt(id: string): void {
+	store()?.removeItem(keyFor(id));
+}