Add session checkpoint notes for the encrypted-vault work

CHECKPOINT-encryption.md

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+# Checkpoint — Password-encrypted vault (2026-08-03)
+
+A stopping point you can return to if this session is closed. Everything below was
+verified green at the moment this file was written.
+
+## Where things are
+
+- Project: `/home/avi/Projects/skills/nost-feed-manager`
+- Git repo: `master` @ `7e3bac3` ("Add Nostr Feed Manager: Rust backend with Electron + React GUI")
+- The encryption work is **uncommitted** — all changes are in the working tree.
+- Also relevant: `/home/avi/Projects/nostr_backend/nostr_backendmanager.md` (old-CLI docs, untouched),
+  and `/home/avi/Projects/nostr_backend/vlog-website/` (separate, untouched).
+
+## What was completed: password-encrypted vault
+
+- Secret keys are now encrypted at rest with **AES-256-GCM** under a key derived via **Argon2id**
+  from the user's password. Vaults stay plaintext until a password is set (opt-in).
+- Encryption only covers the secret keys; labels/npubs stay readable so profiles can be browsed
+  while the vault is locked. The derived key lives only in memory for the session.
+
+## Files changed (19 modified, 4 new)
+
+Modified:
+- `Cargo.toml`, `Cargo.lock` — added `argon2`, `aes-gcm`, `base64`, `getrandom`, `rpassword`
+- `README.md` — documented the new feature + CLI commands
+- `src/lib.rs`, `src/app.rs`, `src/vault.rs`, `src/profiles.rs`, `src/publish.rs`,
+  `src/ipc.rs`, `src/main.rs`, `src/errors.rs`
+- `frontend/src/App.tsx`, `frontend/src/lib/api.ts`, `frontend/src/lib/types.ts`,
+  `frontend/src/state/AppProvider.tsx`, `frontend/src/screens/SettingsScreen.tsx`,
+  `frontend/src/styles.css`, `frontend/src/test/apiMock.ts`, `frontend/src/test/fakeBackend.ts`
+
+New:
+- `src/crypto.rs` — Argon2id KDF + AES-256-GCM encrypt/decrypt + password verifier
+- `frontend/src/components/UnlockModal.tsx`
+- `frontend/src/components/VaultPasswordModal.tsx`
+- `frontend/src/test/VaultPassword.test.tsx`
+
+## New backend API (IPC + CLI)
+
+IPC methods: `set_vault_password { current_password?, new_password }`,
+`unlock_vault { password }`, `lock_vault`, `remove_vault_password { password }`.
+
+CLI: `set-password`, `remove-password`, `unlock`; `create`/`publish` auto-prompt when locked.
+Passwords come from `NFM_PASSWORD` env var or a hidden terminal prompt — never argv.
+Min password length: 8 chars.
+
+## How it was verified (all green)
+
+```
+cargo test            # 50 passed
+cargo clippy --all-targets   # clean
+cargo fmt --check     # clean
+cargo build --release # builds
+npm run typecheck     # clean (frontend/)
+npm run lint          # clean (pre-existing module warning only)
+npm run format:check  # clean
+npm test              # 52 passed (10 files)
+```
+
+Plus a manual end-to-end CLI smoke test: create → set-password → vault file shows only base64
+ciphertext → wrong password rejected → correct password creates encrypted profile. Temp data
+was cleaned up (`/tmp/nfm-e2e` removed).
+
+## How to resume
+
+1. Open the repo: `cd /home/avi/Projects/skills/nost-feed-manager`
+2. Inspect the diff: `git diff` (work is still uncommitted)
+3. To try it: `cargo build --release` then
+   `XDG_DATA_HOME=/tmp/nfm-smoke ./target/release/nostr-manager-backend create "Alice"`,
+   `NFM_PASSWORD=... ./target/release/nostr-manager-backend set-password`
+4. Re-run verification with the commands above.
+
+## Outstanding / next steps (if you continue)
+
+- Decide whether to **commit** the work (nothing is committed yet).
+- `nostr_backendmanager.md` still lists "password-based vault encryption" as a future item and was
+  left untouched — it may deserve updating to match reality.
+- No lock-screen gate: browsing works while locked; only create/publish require unlocking (intended).