Implement Download to Device

## Description

Implement actual device download functionality to transfer waveform data to oscilloscope (Arb1-Arb4 storage locations).

## Current Status

- [x] Download button exists in UI
- [ ] Download functionality not implemented (currently shows alert)

## Files to Modify

- `src/renderer/api/ui-manager.js` - Replace alert at line 286 with actual implementation
- `src/preload/index.js` - Add device communication IPC channel
- `src/main/index.js` - Add device download handler with location selection (Arb1-Arb4)

## Implementation Details

This requires hardware-specific communication protocol (USB/Serial). May need external library like `node-serialport`.

## Priority
Low - Requires hardware access

## Acceptance Criteria
- Download button initiates device communication
- User can select storage location (Arb1-Arb4)
- Waveform data transfers to selected location
- Success/failure feedback provided to user
- Error handling for connection issues