feat(desktop): add password-protected backups in settings

[tellaho]

Category: new-feature
User Impact: Users can create, download, and verify a password-protected backup of their private identity from desktop Settings.

Problem: Buzz does not currently give signed-in users a Settings-based path to protect or validate their private identity independently of onboarding. Solution: Add a focused backup menu to the private-key row, keep encryption and verification local in Rust, and preserve completed encrypted backups briefly so native saves can be retried without repeating encryption.

File changes

desktop/src/features/settings/
Adds the background backup lifecycle, create and test dialogs, private-key menu integration, password handling, and focused unit coverage.

desktop/src/features/onboarding/ui/NsecMaskedDisplay.tsx
Extends the masked private-key display with reusable overflow-menu actions used by Settings.

desktop/src/app/App.tsx
Mounts the backup provider at app scope so encryption and save work survive closing Settings or the modal.

desktop/src/shared/api/tauriIdentity.ts
Adds typed desktop bindings for local backup creation, save, selection, and verification.

desktop/src-tauri/src/key_backup.rs and desktop/src-tauri/src/commands/identity.rs
Implements local NIP-49 encryption, password generation, file handling, and public-identity-only verification results.

desktop/src-tauri/src/egress_guard.rs and guarded call sites
Blocks encrypted secret material from relay, websocket, snapshot, sharing, and huddle egress paths.

desktop/src-tauri tests and fixtures
Covers encryption, verification, file behavior, and fail-closed no-egress protections.

desktop/src/testing/e2eBridge.ts, desktop/tests/, and desktop/playwright.config.ts
Expands the mock native bridge and browser coverage across create, retry, expiry, and current/different-identity verification states.

desktop/src-tauri/Cargo.toml, Cargo.lock, and assets
Adds the local cryptography/password-generation dependencies and embedded short-word list.

Reproduction steps

1. Run the desktop app and open Settings → Profile → Identity.
2. Open the private-key overflow menu and choose Create backup.
3. Enter or generate a valid password, submit, and confirm progress continues if the dialog or Settings is closed.
4. Save the resulting .ncryptsec file; cancel and retry to confirm the temporary download remains available.
5. Choose Test backup, select the file, enter a wrong password, then retry with the correct password.
6. Confirm success identifies whether the backup matches the current identity and displays only the public npub.

Screenshots

Settings identity	Private-key menu	Create backup

Encrypting	Download available	Test success

Visual review and additional states: Buzz thread

[tellaho]

Resolved the blocking NIP-49 verification finding and rebased onto current main.

• Untrusted backups now have their embedded log_n checked before EncryptedSecretKey::decrypt can invoke scrypt. Costs above 18 return an explicit unsupported-cost error.
• Added boundary coverage proving log_n=18 remains accepted and log_n=19 is rejected before decryption.
• Resolved the main integration overlap while retaining both the new linux_media wiring and backup/security modules.
• Rebased head: 329f20e463a41f28a6ab3dfbe43ed0fb46c0d319.

Independent review confirmed the resource ceiling is on the only untrusted decrypt path and upstream-compatible backups at or below the ceiling still work. Full frontend/Rust suites and focused Settings/security tests passed locally; all PR checks are now green.

Two previously noted items remain non-blocking follow-ups: the stale BACKUP_FILE_NAME doc comment, and rust-nostr's normalized password/derived-key locals not currently using Zeroizing (upstream-library behavior).