Score channels by RSSI and adjacent-channel overlap

[vpadman1]

Summary

Previously `recommend_channel` used raw network counts — treated one strong nearby AP the same as five weak ones and ignored 2.4 GHz adjacent-channel interference.

Now the recommender weights each nearby network by signal strength and, on 2.4 GHz, by distance from the candidate channel.

• `score_channels(networks, channels, band)` — returns a congestion score per channel, lower is better.
  • RSSI buckets: strong (> −55 dBm) = 3.0, medium (−55..−75) = 1.5, weak (< −75) = 0.5.
  • 2.4 GHz adjacent-channel spill: 1.0 at Δ=0, 0.5 at Δ=±1..±2, 0.25 at Δ=±3..±4, 0 beyond. Models the overlap between adjacent 2.4 GHz channels.
  • 5 GHz channels at 20 MHz don't overlap, so no spill is applied — 5 GHz path is O(n) via dict lookup.
• `recommend_channel` picks the lowest-scoring channel (same current-channel tiebreaker as before). The dashboard chart still displays raw counts since counts are more intuitive than weighted scores.
• 4 new tests covering strong-vs-weak weighting, 2.4 GHz spill, 5 GHz no-overlap, and empty input.

Test plan

• `uv run pytest -v` — 32/32 passing (28 existing + 4 new)
• `uvx ruff check .` — clean
• CI green