feat: draft settings persistence — shared model/thinking/context record for the new-session surfaces

Both new-session pages (the home graph view's topic dock and /chat/new)
held their model + thinking + knowledge-context choices in plain
useState(undefined) — every navigation reset them to config defaults.

Now both surfaces read/write ONE server-side record
(data/draft-settings.json via GET/PUT /api/settings/draft, the
render-settings pattern): whatever the learner last left the controls on
is what every later visit shows. useDraftSettings loads on mount with a
touch-guard (a slow GET never clobbers a fast choice), saves
optimistically. Session creation still carries the choice into the new
chat exactly as before; per-session settings are untouched.

The PUT validation is the exact per-chat contract — parseSettingsBody
extracted verbatim from app/api/chats/[id]/settings and shared by both
routes (unknown model/level → 400 with the supported list, never a
silent clamp). 400s on garbage, {} = the clear-overrides write.

Live-verified: Cmd+K+Enter model pick, Shift+Tab thinking cycle, context
slider — all held across home → session → home → /chat/new, and carried
into a created session's record.

components/draft-settings.ts

@@ -0,0 +1,62 @@
+"use client";
+
+/**
+ * The DRAFT (pre-session) settings hook — the shared persistence behind
+ * the two new-session surfaces (the home graph view's topic dock and
+ * /chat/new). The model + thinking + knowledge-context choices live in
+ * ONE server-side record (GET/PUT /api/settings/draft →
+ * data/draft-settings.json), so whatever the learner last left the
+ * controls on is what every later visit shows (owner, draft-settings wave
+ * 2026-09-18): navigating away and back keeps the feel. Per-session
+ * settings are a different record (the chat's own) — untouched here.
+ *
+ * Contract: settings === undefined until the first GET resolves — the
+ * controls render config-pinned defaults in that window, same as today.
+ * The GET is touch-guarded: if the learner changes something before the
+ * load lands, the loaded copy is dropped, never clobbering the choice.
+ * Saves are optimistic (the controls only ever send valid values — the
+ * picker offers catalog models, the slider sends its stops) and
+ * fire-and-forget; a failed write logs, and the next mount falls back to
+ * the persisted truth.
+ */
+
+import { useCallback, useEffect, useRef, useState } from "react";
+import type { ChatSettings } from "@/src/core";
+
+export function useDraftSettings(): {
+	settings: ChatSettings | undefined;
+	save: (settings: ChatSettings) => void;
+} {
+	const [settings, setSettings] = useState<ChatSettings | undefined>(undefined);
+	const touchedRef = useRef(false);
+
+	useEffect(() => {
+		let cancelled = false;
+		fetch("/api/settings/draft")
+			.then((res) => (res.ok ? (res.json() as Promise<ChatSettings | null>) : null))
+			.then((loaded) => {
+				// Never clobber a choice made while the load was in flight.
+				if (!cancelled && loaded !== null && !touchedRef.current) setSettings(loaded);
+			})
+			.catch(() => {
+				/* transient — defaults stand, the next change rewrites the record */
+			});
+		return () => {
+			cancelled = true;
+		};
+	}, []);
+
+	const save = useCallback((next: ChatSettings) => {
+		touchedRef.current = true;
+		setSettings(next);
+		void fetch("/api/settings/draft", {
+			method: "PUT",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify(next),
+		}).catch((err: unknown) => {
+			console.error("draft settings write failed:", err);
+		});
+	}, []);
+
+	return { settings, save };
+}