fix: finish new-session opening-message handoff

components/start-session.ts

@@ -0,0 +1,34 @@
+import { clearOpeningPrompt, OPENING_PROMPT_FAILURE, stashOpeningPrompt } from "./opening-prompt";
+
+interface SessionCreation {
+	title: string;
+	kind?: "practice";
+	graphContext?: { propositions: readonly { id: string; name: string; statement: string }[] };
+}
+
+/** Per source-composer instance. Retain a successful creation if storage
+ * fails, so an unchanged retry does not create another empty session.
+ * Changed text/context/settings is a different start, not a stale title.
+ */
+export class SessionStarter {
+	private created?: { identity: string; id: string };
+
+	async start(body: SessionCreation, fetchLike: typeof fetch, settingsIdentity?: string): Promise<string> {
+		const identity = JSON.stringify([body, settingsIdentity]);
+		if (this.created?.identity !== identity) {
+			if (this.created) clearOpeningPrompt(this.created.id);
+			this.created = undefined;
+			const res = await fetchLike("/api/chats", {
+				method: "POST",
+				headers: { "Content-Type": "application/json" },
+				body: JSON.stringify(body),
+			});
+			if (!res.ok) throw new Error(OPENING_PROMPT_FAILURE);
+			const result = await res.json() as { id?: unknown };
+			if (typeof result.id !== "string" || !/^[A-Za-z0-9_-]+$/.test(result.id)) throw new Error(OPENING_PROMPT_FAILURE);
+			this.created = { identity, id: result.id };
+		}
+		if (!stashOpeningPrompt(this.created.id, body.title)) throw new Error(OPENING_PROMPT_FAILURE);
+		return `/chat/${this.created.id}`;
+	}
+}