Backend: notifications now show a plain "-" for an absent name, vendor, or
device type (was empty string / "(unknown)" / "Unknown"), via a single
UNKNOWN_PLACEHOLDER constant.
Frontend:
- Empty/unknown values render as an em dash everywhere, centralised in a new
Placeholders.emptyValue constant (replaces inline '—' and '(unknown)').
- Route paths moved to a new Routes class, used by the router and every
navigation call site.
- Device event type modelled as a DeviceEventType enum mirroring the backend
(NewDevice/DeviceSeen) instead of bare string comparisons.
- Hardcoded EdgeInsets/SizedBox spacing replaced with existing Insets tokens.
Tests and formatting updated; all backend and frontend tests pass.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
