geolocation: add distance conversion to geoprobe-target-sender and fix pubkey logging in geoprobe-agent

[ben-dz]

Resolves: #3403

Summary of Changes

• Add RTT-to-distance conversion in geoprobe-target-sender output (text and JSON), matching geoprobe-target's existing Max Distance display
• Fix geoprobe-agent log statements to call .String() on pubkeys so they render as base58 instead of byte arrays

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	2	+50 / -19	+31

All changes are core logic across two binaries.

Key files (click to expand)

• controlplane/telemetry/cmd/geoprobe-target-sender/main.go — add speed-of-light constants, calculateMaxDistance function, max_distance_miles/max_distance_km fields to JSON structs, and Max Distance line to text output
• controlplane/telemetry/cmd/geoprobe-agent/main.go — append .String() to 5 pubkey log values so they serialize as base58

Testing Verification

• go test ./controlplane/telemetry/cmd/geoprobe-target-sender/... passes
• golangci-lint run clean on both packages
• Distance formula and constants match the existing implementation in geoprobe-target