feat(backend): persistent settings store + settingsFile config

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

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;
+}