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.

src/store/draft-settings.ts

@@ -0,0 +1,45 @@
+/**
+ * Server-side persistence for the DRAFT (pre-session) settings — the
+ * model + thinking-level + knowledge-context choices made on the
+ * new-session surfaces (the home graph view's topic dock and /chat/new).
+ * One small JSON blob: data/draft-settings.json, read-modify-write.
+ *
+ * Both surfaces share ONE record (owner, draft-settings wave 2026-09-18):
+ * whatever the learner last left the controls on is what the next visit
+ * shows — navigating away and back must not reset the feel. Per-session
+ * settings live in the chat record (PUT /api/chats/:id/settings) and are
+ * untouched by this.
+ *
+ * This module only moves bytes: the file is written exclusively through the
+ * validated route (app/api/settings/draft — parseSettingsBody +
+ * validateChatSettings gate every write), so no sanitizer runs here. On
+ * read, anything unreadable/corrupt is null = first boot = config defaults.
+ */
+
+import fs from "node:fs";
+import path from "node:path";
+import { DRAFT_SETTINGS_FILE } from "../../config.ts";
+import type { ChatSettings } from "../core";
+
+/** The persisted draft settings, or null when never saved / unreadable. */
+export function readDraftSettings(): ChatSettings | null {
+	let raw: string;
+	try {
+		raw = fs.readFileSync(DRAFT_SETTINGS_FILE, "utf-8");
+	} catch {
+		return null; // never saved / unreadable → the surfaces use config defaults
+	}
+	try {
+		const parsed: unknown = JSON.parse(raw);
+		if (parsed === null || typeof parsed !== "object" || Array.isArray(parsed)) return null;
+		return parsed as ChatSettings;
+	} catch {
+		return null; // corrupt → defaults, never crash the surfaces
+	}
+}
+
+/** Persist the validated settings verbatim. */
+export function writeDraftSettings(settings: ChatSettings): void {
+	fs.mkdirSync(path.dirname(DRAFT_SETTINGS_FILE), { recursive: true });
+	fs.writeFileSync(DRAFT_SETTINGS_FILE, JSON.stringify(settings, null, "\t"), "utf-8");
+}