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 };
+}

components/home.tsx

@@ -5,6 +5,7 @@ import { useRouter } from "next/navigation";
 import { CornerDownLeftIcon } from "lucide-react";
 import { GraphView } from "@/components/graph-view";
 import { GraphContextControl, effectiveContextTokens } from "@/components/graph-context-control";
+import { useDraftSettings } from "@/components/draft-settings";
 import { DictationBar, useDictation } from "@/components/dictation";
 import { SidebarToggle } from "@/components/sidebar";
 import { StatsBar } from "@/components/stats-bar";
@@ -25,10 +26,13 @@ export function Home() {
 		append: (t) => setTopic((v) => (v ? v.replace(/\s+$/, "") + " " + t : t)),
 	});
 	const [busy, setBusy] = useState(false);
-	// The model+thinking selection made pre-chat — lifted from the StatsBar's
-	// ModelControl, carried to the chat POST then PUT-on-create (the model
-	// picker defers its save when chatId is empty; see components/model-picker).
-	const [settings, setSettings] = useState<ChatSettings | undefined>(undefined);
+	// The model+thinking+context selection made pre-chat — the SHARED draft
+	// record (draft-settings wave, 2026-09-18): both new-session surfaces
+	// (this topic dock and /chat/new) read and write ONE persisted choice,
+	// so what the learner last left the controls on is what shows on return.
+	// Still carried to the chat POST then PUT-on-create (the model picker
+	// defers its save when chatId is empty; see components/model-picker).
+	const { settings, save: saveDraft } = useDraftSettings();
 
 	const start = async () => {
 		const t = topic.trim();
@@ -140,13 +144,11 @@ export function Home() {
 						turns={[] as TurnUsage[]}
 						chatId=""
 						settings={settings}
-						onSettingsSaved={setSettings}
+						onSettingsSaved={saveDraft}
 						leftSlot={
 							<GraphContextControl
 								value={effectiveContextTokens(settings)}
-								onChange={(tokens) =>
-									setSettings((s) => ({ ...s, graphContextTokens: tokens }))
-								}
+								onChange={(tokens) => saveDraft({ ...settings, graphContextTokens: tokens })}
 							/>
 						}
 					/>

components/new-session.tsx

@@ -6,6 +6,7 @@ import { Conversation, ConversationContent, ConversationEmptyState } from "@/com
 import { Composer } from "@/components/composer";
 import { DictationBar, useDictation } from "@/components/dictation";
 import { GraphContextControl, effectiveContextTokens } from "@/components/graph-context-control";
+import { useDraftSettings } from "@/components/draft-settings";
 import { PhaseChip } from "@/components/phase-divider";
 import { SidebarToggle } from "@/components/sidebar";
 import { StatsBar } from "@/components/stats-bar";
@@ -150,9 +151,13 @@ export function NewSession() {
 	const router = useRouter();
 	const [text, setText] = useState("");
 	const [busy, setBusy] = useState(false);
-	// The pre-chat model+thinking selection, lifted from the StatsBar's
-	// ModelControl (empty chatId → it never PUTs; it hands the choice up).
-	const [settings, setSettings] = useState<ChatSettings | undefined>(undefined);
+	// The pre-chat model+thinking+context selection — the SHARED draft
+	// record (draft-settings wave, 2026-09-18): /chat/new and the home
+	// topic dock read and write ONE persisted choice, so what the learner
+	// last left the controls on is what shows on return. ModelControl with
+	// an empty chatId never PUTs — it hands the choice up; it still rides
+	// the session POST then PUT-on-create.
+	const { settings, save: saveDraft } = useDraftSettings();
 
 	const send = useCallback(() => {
 		if (!text.trim() || busy) return;
@@ -172,7 +177,7 @@ export function NewSession() {
 			onChange={setText}
 			onSend={send}
 			settings={settings}
-			onSettingsSaved={setSettings}
+			onSettingsSaved={saveDraft}
 		/>
 	);
 }