fix(channelplan): score current DFS channel and clarify empty state

[gnacho]

The channel plan page could show nothing or absurd values:

• When the router's current channel is DFS (52/100/112/116...), it is not in the candidate list (no-DFS only), so currentScore stayed at MaxInt and the UI painted 9223372036854775807.
• The page opened with the first router (gateway), which since running NetGrip does not report scans; the empty state did not explain why.

Changes:

• Recommend now always computes the score of the current channel (informative) even when it is not a candidate.
• Clearer empty state (ES/EN) explaining that NetGrip routers do not scan and pointing to routers with the NetPulse agent.

Closes #518