feat: Introduce env variables encryption & auto-discover MFP session cookies

[rencsaridogan]

Add Fernet encryption + OS keychain support for credentials

Summary

• Adds optional at-rest encryption for MFP_USERNAME / MFP_PASSWORD using Fernet symmetric encryption (cryptography library)
• MFP_SECRET_KEY is resolved at runtime from two sources in order: MFP_SECRET_KEY env var, then OS keychain (mfp-mcp / MFP_SECRET_KEY) — the key never has to appear in the Claude Desktop config
• Adds get_secret_key() that checks env then calls keyring.get_password(), used by get_decrypted_credential() transparently
• Adds scripts/store-key.ts — a one-time Node.js CLI (npm run store-key) that generates a Fernet-compatible key, stores it in the OS keychain, and prints the encryption snippet; zero manual keychain commands needed
• Adds cryptography>=41.0.0 as a Python runtime dependency (keyring was already present)
• Adds package.json / tsconfig.json for the Node tooling; node_modules/ and package-lock.json are gitignored
• Updates README with a 3-step encrypted credentials guide, a full Key Management CLI reference, and an updated Project Structure tree
• Add Chromium browser auto-discovery as a primary auth path on macOS — works with Arc, Chrome, Edge, Brave, Vivaldi, Opera, or any other installed Chromium-based browser
• Fix authenticate_with_credentials false-positive success check that was poisoning ~/.mfp_mcp/cookies.json with pre-auth garbage on every call

Why

Credentials in claude_desktop_config.json sit in plain text. Encrypting them is only meaningful if the key is stored separately — otherwise a leaked config still exposes everything. By keeping MFP_SECRET_KEY in the OS keychain and out of the config entirely, a leaked config yields useless ciphertext. store-key makes this a one-command setup with no manual keychain interaction.

How it works (env variables encryption)

1. npm run store-key — generates key, writes to keychain, prints key + Python encrypt snippet
2. User encrypts MFP_USERNAME / MFP_PASSWORD using the printed snippet
3. Encrypted values go into Claude Desktop config; key stays in keychain
4. On startup, get_secret_key() checks env var first, then keyring.get_password("mfp-mcp", "MFP_SECRET_KEY")
5. get_decrypted_credential() decrypts values before passing to auth flow
6. If no key is found anywhere, raw env values are used — fully backward compatible

How it works (chromium cookie detection)

1. Chromium auto-discovery (new Method 3 in get_mfp_client)

try_chromium_browsers_for_session_cookies():

1. Enumerates macOS keychain entries matching * Safe Storage
2. Derives each browser's AES-128 key (PBKDF2-HMAC-SHA1, salt=saltysalt, 1003 iters — Chromium's os_crypt_mac.mm scheme)
3. Locates and copies the browser's cookies SQLite DB
4. Decrypts v10/v11 entries, stripping the 32-byte SHA-256 host_key prefix that modern Chromium prepends to plaintext
5. Returns the first browser with a real MFP session token (session-token or _mfp_session)

Cookies are only persisted to ~/.mfp_mcp/cookies.json after a verified round-trip to MFP, so a transient failure can't poison the file.

2. Strict success check in authenticate_with_credentials

The old check matched any cookie containing user/session/auth/logged_in as a substring — too loose. Replaced with _has_real_mfp_session() which requires a session-token substring or _mfp_session cookie. On failure, raises with a clear message pointing users at the auto-discovery path.

3. refresh_browser_cookies tool

• New auto mode (now the default) — runs Chromium discovery
• Explicit aliases: arc, chrome, chromium, edge, brave, vivaldi, opera
• Legacy firefox path via browser_cookie3

Test plan

• npm run store-key generates key and stores it; second run shows the guard prompt
• npm run store-key -- --show prints the stored key
• npm run store-key -- --overwrite replaces existing key
• npm run store-key -- --delete removes it; server falls back to plain-text auth
• Plain-text auth works with no MFP_SECRET_KEY set anywhere (backward compat)
• Auth works with key in MFP_SECRET_KEY env var
• Auth works with key in keychain only (no env var)
• Wrong key logs Decryption failed and falls through to cookie auth
• Keychain backend failure logs a warning and falls through gracefully
• Module imports cleanly (python -c "import mfp_mcp.server")
• End-to-end: wipe cookies.json → call get_mfp_client() with env creds → form login raises cleanly → Chromium discovery finds Arc → 16 cookies extracted with valid __Secure-next-auth.session-token → diary round-trip succeeds → cookies persisted
• authenticate_with_credentials no longer overwrites cookies.json on failure