fix: finalize durable Pi lifecycle

backend/src/settings/settings-store.ts

@@ -22,6 +22,11 @@ export interface SettingsDurability {
   syncDirectory(directory: string): void;
 }
 
+export interface SettingsSnapshot {
+  exists: boolean;
+  rawBase64: string;
+}
+
 /**
  * Settings files are installation defaults only. Personal workspace/model/thinking choices
  * belong to the browser and must never be written back here by request handlers.
@@ -39,6 +44,45 @@ export function loadSettings(cfg: AppConfig): Settings {
   }
 }
 
+/** Capture exact file existence and bytes so host-side configuration can compensate losslessly. */
+export function captureSettingsSnapshot(cfg: AppConfig): SettingsSnapshot {
+  try {
+    return { exists: true, rawBase64: readFileSync(cfg.settingsFile).toString("base64") };
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code === "ENOENT") {
+      return { exists: false, rawBase64: "" };
+    }
+    throw error;
+  }
+}
+
+/** Restore a previously captured settings file exactly, including the clean absent state. */
+export function restoreSettingsSnapshot(
+  cfg: AppConfig,
+  snapshot: SettingsSnapshot,
+  durability: SettingsDurability = defaultDurability,
+): void {
+  if (typeof snapshot.exists !== "boolean" || typeof snapshot.rawBase64 !== "string") {
+    throw new Error("invalid settings snapshot");
+  }
+  const raw = Buffer.from(snapshot.rawBase64, "base64");
+  if (raw.toString("base64") !== snapshot.rawBase64 || (!snapshot.exists && raw.length !== 0)) {
+    throw new Error("invalid settings snapshot");
+  }
+  const directory = dirname(cfg.settingsFile);
+  mkdirSync(directory, { recursive: true });
+  if (snapshot.exists) {
+    replaceSettingsFile(cfg.settingsFile, raw);
+  } else {
+    try {
+      unlinkSync(cfg.settingsFile);
+    } catch (error) {
+      if ((error as NodeJS.ErrnoException).code !== "ENOENT") throw error;
+    }
+  }
+  durability.syncDirectory(directory);
+}
+
 /** Persist settings (pretty JSON). Creates the parent directory if needed. */
 export function saveSettings(
   cfg: AppConfig,