feat(backend): persistent settings store + settingsFile config

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/config.ts

@@ -3,6 +3,7 @@ export interface AppConfig {
   authMode: "none" | "mock" | "oidc";
   defaults: { provider?: string; model?: string; thinking?: string };
   maxPiProcesses: number;
+  settingsFile: string;
 }
 export function loadConfig(env: Record<string, string | undefined>): AppConfig {
   return {
@@ -13,5 +14,6 @@ export function loadConfig(env: Record<string, string | undefined>): AppConfig {
     authMode: (env.AUTH_MODE as AppConfig["authMode"]) ?? "none",
     defaults: { provider: env.PI_PROVIDER, model: env.PI_MODEL, thinking: env.PI_THINKING },
     maxPiProcesses: Number(env.MAX_PI_PROCESSES ?? 4),
+    settingsFile: env.SETTINGS_FILE ?? "data/settings.json",
   };
 }

backend/src/settings/settings-store.ts

@@ -0,0 +1,29 @@
+import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { dirname } from "node:path";
+import type { AppConfig } from "../config.js";
+
+export interface Settings {
+  workspace?: string;
+  provider?: string;
+  model?: string;
+  thinking?: string;
+}
+
+/** Read settings from cfg.settingsFile. Returns {} if missing or invalid. */
+export function loadSettings(cfg: AppConfig): Settings {
+  try {
+    const raw = readFileSync(cfg.settingsFile, "utf8");
+    const parsed = JSON.parse(raw);
+    if (parsed && typeof parsed === "object") return parsed as Settings;
+    return {};
+  } catch {
+    return {};
+  }
+}
+
+/** Persist settings (pretty JSON). Creates the parent directory if needed. */
+export function saveSettings(cfg: AppConfig, s: Settings): Settings {
+  mkdirSync(dirname(cfg.settingsFile), { recursive: true });
+  writeFileSync(cfg.settingsFile, JSON.stringify(s, null, 2) + "\n", "utf8");
+  return s;
+}