feat(security): encrypt persisted provider credentials

Protect every remembered provider credential with Electron safeStorage and restore it only inside the main process. Migrate legacy plaintext settings atomically across the primary config and its backup while keeping credentials memory-only when encryption is unavailable or remembering is disabled.

Initialize credential protection after app readiness but before settings are loaded, add masked renderer projection metadata, and cover encryption, migration, unavailable-storage, and persistence behavior with focused tests.

tests/storage.test.ts

@@ -1,15 +1,24 @@
 import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
-import { afterEach, describe, expect, it } from "vitest";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import { parseDebridLinkApiKeys } from "../src/shared/debrid-link-keys";
 import { getMegaDebridAccountId } from "../src/shared/mega-debrid-accounts";
 import { getProviderUsageDayKey } from "../src/shared/provider-daily-limits";
 import { AppSettings } from "../src/shared/types";
-import { defaultSettings } from "../src/main/constants";
+import { defaultSettings } from "../src/main/constants";
+import { configureCredentialProtector } from "../src/main/credential-protection";
 import { addHistoryEntryForRetention, createStoragePaths, emptySession, loadHistory, loadHistoryForRetention, loadSession, loadSettings, normalizeLoadedSession, normalizeSettings, resetHistoryForRetention, saveHistory, saveSession, saveSessionAsync, saveSettings } from "../src/main/storage";
 
-const tempDirs: string[] = [];
+const tempDirs: string[] = [];
+
+beforeEach(() => {
+  configureCredentialProtector({
+    isEncryptionAvailable: () => true,
+    encryptString: (value) => Buffer.from(value, "utf8").reverse(),
+    decryptString: (value) => Buffer.from(value).reverse().toString("utf8")
+  });
+});
 
 afterEach(() => {
   for (const dir of tempDirs.splice(0)) {
@@ -163,7 +172,7 @@ describe("settings storage", () => {
     expect(loaded.allDebridToken).toBe("");
   });
 
-  it("persists provider credentials when rememberToken is enabled", () => {
+  it("persists provider credentials when rememberToken is enabled", () => {
     const dir = fs.mkdtempSync(path.join(os.tmpdir(), "rd-store-"));
     tempDirs.push(dir);
     const paths = createStoragePaths(dir);
@@ -183,8 +192,30 @@ describe("settings storage", () => {
     expect(loaded.megaLogin).toBe("mega-user");
     expect(loaded.megaPassword).toBe("mega-pass");
     expect(loaded.bestToken).toBe("best-token");
-    expect(loaded.allDebridToken).toBe("all-token");
-  });
+    expect(loaded.allDebridToken).toBe("all-token");
+  });
+
+  it("migrates remembered plaintext provider values without retaining plaintext in config backups", () => {
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "rd-store-"));
+    tempDirs.push(dir);
+    const paths = createStoragePaths(dir);
+    const value = "legacy-value-to-migrate";
+    fs.writeFileSync(paths.configFile, JSON.stringify({
+      ...defaultSettings(),
+      rememberToken: true,
+      token: value
+    }), "utf8");
+
+    const loaded = loadSettings(paths);
+    const config = fs.readFileSync(paths.configFile, "utf8");
+    const backup = fs.existsSync(`${paths.configFile}.bak`)
+      ? fs.readFileSync(`${paths.configFile}.bak`, "utf8")
+      : "";
+
+    expect(loaded.token).toBe(value);
+    expect(config).not.toContain(value);
+    expect(backup).not.toContain(value);
+  });
 
   it("normalizes invalid enum and numeric values", () => {
     const normalized = normalizeSettings({