Session profiles: save & load multiple device/slave configurations

[takker99]

Priority: P3
Category: UX / Productivity

Background:
Users often switch between multiple Modbus devices/slaves with different baud, parity, polling sets, and read/write presets. Manual re-entry is error prone.

Goal:
Provide session profile save/load: persist serial + Modbus settings, commonly used read requests, and optionally last N write templates.

Acceptance Criteria:

• Save Profile: Name + snapshot of current settings (serial config, protocol, last used function codes & addresses, monitor interval)
• List Profiles: stored in localStorage (or IndexedDB) with delete/rename
• Load applies settings immediately (without auto-connecting unless user opts-in)
• Export/Import JSON for sharing (optional nice-to-have)
• Tests: create -> load -> delete cycle; malformed import rejected

Tasks:

1. Define profile schema versioned (v1)
2. Storage service (abstraction for future cloud sync)
3. UI: Profile dropdown + Save / Manage dialog
4. JSON export/import (guard size)
5. Tests for CRUD & schema migration placeholder
6. README update (Usage -> Profiles)

Definition of Done:
Profile round-trip works across reloads; tests green.