fix: finalize durable Pi lifecycle

backend/test/settings-store.test.ts

@@ -1,8 +1,13 @@
 import { test, expect } from "vitest";
-import { closeSync, fsyncSync, mkdtempSync, openSync, rmSync, writeFileSync } from "node:fs";
+import { closeSync, existsSync, fsyncSync, mkdtempSync, openSync, readFileSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { loadSettings, saveSettings } from "../src/settings/settings-store.js";
+import {
+  captureSettingsSnapshot,
+  loadSettings,
+  restoreSettingsSnapshot,
+  saveSettings,
+} from "../src/settings/settings-store.js";
 import { loadConfig } from "../src/config.js";
 
 function cfgWith(file: string) {
@@ -70,3 +75,25 @@ test("saveSettings restores the previous file when post-rename directory durabil
     rmSync(dir, { recursive: true, force: true });
   }
 });
+
+test("settings snapshots restore exact absent and empty-file states", () => {
+  const dir = mkdtempSync(join(tmpdir(), "tht-settings-snapshot-"));
+  try {
+    const file = join(dir, "settings.json");
+    const cfg = cfgWith(file);
+    const absent = captureSettingsSnapshot(cfg);
+    expect(absent).toEqual({ exists: false, rawBase64: "" });
+    saveSettings(cfg, { provider: "new", model: "model", thinking: "high" });
+    restoreSettingsSnapshot(cfg, absent);
+    expect(existsSync(file)).toBe(false);
+
+    writeFileSync(file, Buffer.alloc(0), { mode: 0o600 });
+    const empty = captureSettingsSnapshot(cfg);
+    expect(empty.exists).toBe(true);
+    saveSettings(cfg, { provider: "new", model: "model", thinking: "high" });
+    restoreSettingsSnapshot(cfg, empty);
+    expect(readFileSync(file)).toEqual(Buffer.alloc(0));
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});