Create Platform Helper Module (`helpers/platform.js`)

### Summary  
Introduce a helper module to centralise platform detection and build‑type logic.

### Tasks  
- Detect OS (win/mac/linux)  
- Detect Snap environment  
- Detect CI environment  
- Provide helper functions for build scripts  
- Ensure updater can use platform info (#116)

### Branch  
`issue/115-build-system-modernisation` → `issue/119-build-script-redesign`
