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.

app/api/settings/draft/route.ts

@@ -0,0 +1,39 @@
+/**
+ * GET/PUT /api/settings/draft — server persistence for the DRAFT (pre-
+ * session) settings: the model + thinking-level + knowledge-context
+ * selections made on the new-session surfaces (the home graph view's topic
+ * dock and /chat/new). Both surfaces share ONE record (owner, 2026-09-18):
+ * whatever the learner last left the controls on is what the next visit
+ * shows. The file lives at data/draft-settings.json
+ * (config.DRAFT_SETTINGS_FILE).
+ *
+ *   GET → 200 {settings} once saved, 200 null on first boot (the surfaces
+ *         keep config-pinned defaults until the learner chooses something).
+ *   PUT → 200 {settings} — the VALIDATED copy persisted verbatim. The exact
+ *         per-chat contract (parseSettingsBody + the catalog gate): unknown
+ *         provider/model or an unsupported thinking level is a 400 carrying
+ *         the supported list, never a silent clamp.
+ *
+ * Per-session settings (PUT /api/chats/:id/settings) are a separate record
+ * untouched by this — the draft choice still rides along on session
+ * creation exactly as before.
+ */
+import { readDraftSettings, writeDraftSettings, parseSettingsBody, validateChatSettings } from "@/src/server";
+
+export const dynamic = "force-dynamic";
+
+export async function GET() {
+	return Response.json(readDraftSettings());
+}
+
+export async function PUT(request: Request) {
+	const body = await request.json().catch(() => null);
+	const parsed = parseSettingsBody(body);
+	if (!parsed.ok) return Response.json({ error: parsed.error }, { status: 400 });
+	const verdict = validateChatSettings(parsed.settings);
+	if (!verdict.ok) {
+		return Response.json({ error: verdict.error, supported: verdict.supported }, { status: 400 });
+	}
+	writeDraftSettings(parsed.settings);
+	return Response.json({ settings: parsed.settings });
+}