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.

tests/draft-settings.test.ts

@@ -0,0 +1,108 @@
+/**
+ * Tests for the draft-settings wave (2026-09-18): the shared settings-body
+ * parser (the one seam behind BOTH settings routes — per-chat and draft —
+ * so their parse behavior is identical) and the draft-settings store
+ * (data/draft-settings.json read/write, first-boot null semantics).
+ *
+ * The store tests write the real repo data file (npm test's shared data/,
+ * disposable scratch by doctrine) and clean up after themselves.
+ */
+
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import { afterEach, describe, it } from "node:test";
+import { GRAPH_CONTEXT_TOKENS_MAX, type ChatSettings } from "../src/core";
+import { parseSettingsBody } from "../src/server/settings-body.ts";
+import { DRAFT_SETTINGS_FILE } from "../config.ts";
+import { readDraftSettings, writeDraftSettings } from "../src/store/draft-settings.ts";
+
+describe("parseSettingsBody (shared by both settings routes)", () => {
+	it("an empty object parses to {} — the clear-overrides write", () => {
+		const r = parseSettingsBody({});
+		assert.ok(r.ok);
+		assert.deepEqual(r.ok ? r.settings : null, {});
+	});
+
+	it("a full valid body parses field-for-field", () => {
+		const r = parseSettingsBody({
+			model: { provider: "openrouter", id: "z-ai/glm-5.3-flash" },
+			thinkingLevel: "high",
+			graphContextTokens: 256,
+		});
+		assert.ok(r.ok);
+		assert.deepEqual(r.settings, {
+			model: { provider: "openrouter", id: "z-ai/glm-5.3-flash" },
+			thinkingLevel: "high",
+			graphContextTokens: 256,
+		});
+	});
+
+	it("partial bodies keep only the present overrides (replace-whole)", () => {
+		const r = parseSettingsBody({ thinkingLevel: "low" });
+		assert.ok(r.ok);
+		assert.deepEqual(r.settings, { thinkingLevel: "low" });
+	});
+
+	it("rejects non-object bodies", () => {
+		for (const body of ["junk", 42, [{}]]) {
+			const r = parseSettingsBody(body);
+			assert.equal(r.ok, false, JSON.stringify(body));
+			if (!r.ok) assert.match(r.error, /Invalid JSON body/);
+		}
+	});
+
+	it("rejects malformed model and non-string thinkingLevel", () => {
+		assert.equal(parseSettingsBody({ model: "nope" }).ok, false);
+		assert.equal(parseSettingsBody({ model: { provider: 1, id: "x" } }).ok, false);
+		assert.equal(parseSettingsBody({ thinkingLevel: 7 }).ok, false);
+	});
+
+	it("rejects out-of-range or non-integer graphContextTokens — never clamps", () => {
+		for (const bad of [-1, GRAPH_CONTEXT_TOKENS_MAX + 1, 64.5, "256"]) {
+			const r = parseSettingsBody({ graphContextTokens: bad });
+			assert.equal(r.ok, false, String(bad));
+		}
+		const ok = parseSettingsBody({ graphContextTokens: GRAPH_CONTEXT_TOKENS_MAX });
+		assert.equal(ok.ok, true);
+	});
+});
+
+describe("the draft-settings store", () => {
+	afterEach(() => {
+		try {
+			fs.rmSync(DRAFT_SETTINGS_FILE);
+		} catch {
+			/* nothing to clean */
+		}
+	});
+
+	it("reads null on first boot (no file) — the surfaces use config defaults", () => {
+		try {
+			fs.rmSync(DRAFT_SETTINGS_FILE);
+		} catch {
+			/* absent already */
+		}
+		assert.equal(readDraftSettings(), null);
+	});
+
+	it("write then read roundtrips verbatim — validated shape, no sanitizer games", () => {
+		const settings: ChatSettings = {
+			model: { provider: "openrouter", id: "z-ai/glm-5.3-flash" },
+			thinkingLevel: "high",
+			graphContextTokens: 1024,
+		};
+		writeDraftSettings(settings);
+		assert.deepEqual(readDraftSettings(), settings);
+	});
+
+	it("an empty {} record roundtrips (all overrides cleared)", () => {
+		writeDraftSettings({});
+		assert.deepEqual(readDraftSettings(), {});
+	});
+
+	it("a corrupt file reads as null — defaults, never a crash", () => {
+		fs.rmSync(DRAFT_SETTINGS_FILE, { force: true }); // a directory from a broken earlier run also blocks
+		fs.writeFileSync(DRAFT_SETTINGS_FILE, "{ not json", "utf-8");
+		assert.equal(readDraftSettings(), null);
+	});
+});