feat(cli): add ldgr devices command for QR/X25519 device onboarding

[kafkade]

Description

Adds an ldgr devices command group that lets you pair a new device to your vault against a self-hosted ldgr-server, transferring the vault key end-to-end encrypted over the existing sync relay. This wires the already-implemented core onboarding crypto and server relay into an operator-facing CLI surface.

What's included

• ldgr devices subcommands:
  • list — show the devices registered to your vault's account.
  • add — start a pairing session on an already-set-up device; renders a scannable QR code, a copy-paste token, and a verification code, then waits for the new device to join.
  • join <token> — complete onboarding on a new device (one that has already run ldgr sync setup); prints its own verification code and installs the received vault key.
  • remove <id> — deregister a device.
• Transport-agnostic pairing orchestration in ldgr-core (sync::pairing): I/O-free per ADR-005, with the pairing token codec, an ECDH handshake, and zeroize-on-drop session state. All I/O (QR, prompts, poll loops, session storage) lives in the CLI.
• End-to-end encrypted vault-key handoff: the key is AES-256-GCM encrypted under the X25519 ECDH shared secret, so the relay/server only ever sees ciphertext. Both devices display a matching verification code so a man-in-the-middle can be detected. No server changes were needed — the 3-message handoff is mapped onto the relay's 2-leg model via a two-offer handshake (both devices authenticate as the same account).
• Docs: new "Pairing a new device" section in docs/sync-setup.md and a CHANGELOG.md entry.

iOS QR-scanning onboarding remains a planned follow-up; today join takes the token as an argument.

Related Issues

Closes #225

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Refactoring (no functional changes)
• CI / infrastructure
• Other (describe below)

Component

• crates/ldgr-core/ — Core library (crypto, accounting, storage)
• crates/ldgr-cli/ — CLI tool
• crates/ldgr-server/ — Sync server
• bindings/swift/ — UniFFI Swift bindings
• apps/ios/ — iOS / iPadOS / watchOS app
• apps/web/ — Web app (Next.js + WASM)
• docs/ — Documentation

Privacy Checklist

• No plaintext financial data is sent to or stored on the server
• No new metadata exposure introduced (or documented if unavoidable)
• Any new external service interaction is documented in the trust boundary model
• Crypto changes use audited RustCrypto crates — no custom primitives
• Key material is never exposed in error messages, logs, or Debug output

Checklist

• I have read CONTRIBUTING.md
• Tests pass (cargo test --workspace)
• Clippy passes (cargo clippy --workspace -- -D warnings)
• Formatting passes (cargo fmt --check)
• I have updated documentation (if applicable)

---

Testing: adds crates/ldgr-server/tests/device_pairing_e2e.rs with two end-to-end tests proving (1) a joining device recovers the initiator's exact vault key over the encrypted relay and (2) device register/list/remove round-trips. Also adds unit tests for the pairing token codec in ldgr-core. Verified cargo test --workspace --all-features, cargo clippy --workspace --all-features --all-targets -- -D warnings, cargo fmt --check, and markdownlint-cli2 all pass.

CI note: no CI job names changed, so no kafkade/github-infra Terraform update is required.