Plan 1a: Replace MAC-based device ID with persisted UUID

[careck]

Parent: #171 (Mobile support: iOS and Android via Tauri v2)

Summary

Replace the MAC-address-based device ID (mac_address crate) with a persisted UUID. This is a prerequisite for mobile support (iOS/Android don't expose MAC addresses) and fixes existing relay sync instability caused by MAC address drift across network interfaces.

Plan

See docs/superpowers/plans/2026-04-29-device-id-migration.md on the mobile branch.

Key Changes

• New API: get_or_create_seed_device_id(data_dir) + resolve_device_id(conn, data_dir) in device.rs
• Priority chain: workspace_meta → operations table → app-level seed file → generate new UUID
• Migration gate: Block new workspace creation if existing workspaces found but no seed file (forces user to open existing workspace first to extract device ID)
• Remove mac_address dependency entirely from krillnotes-core
• Update 21 call sites in krillnotes-desktop sync/relay commands to use workspace-held device_id instead of computing from MAC

Affects

• krillnotes-core: device.rs, export.rs, workspace/mod.rs, Cargo.toml
• krillnotes-desktop: lib.rs, commands (invites.rs, swarm.rs, relay_accounts.rs, sync.rs, identity.rs, receive_poll.rs)

Acceptance Criteria

• mac_address crate removed from Cargo.toml
• Device ID persists across app restarts (read from file/DB, not computed)
• Existing workspaces preserve their device_id on first open after migration
• New workspaces inherit device_id from seed file
• Migration gate prevents accidental UUID generation when existing workspaces exist
• cargo test -p krillnotes-core passes
• Desktop app runs with no regressions