Problem

Signal indicators are visually inconsistent across the app:

• Scanner/device tiles use one icon mapping.
• SNR indicator surfaces (top bar, neighbors/path views) use a different mapping.

This makes two devices with similar signal quality look different depending on which screen/component rendered the indicator.

Expected behavior

Use one shared signal icon + color tier mapping across RSSI- and SNR-based UI surfaces so users get consistent visual semantics.

Proposed fix

• Introduce a shared signal UI mapper utility.
• Use it from both device_tile.dart and snr_indicator.dart.
• Keep numeric text units unchanged (dBm, dB).

Validation

• Build app successfully.
• Verify scanner list and SNR widgets now use the same icon family and tier colors.