feat: sample the public IP into a cache file

The draw hook must never make a network call: a hung request blocks the
Cairo draw and freezes the dashboard. Thirty minutes, not the old config's
five, because a residential address is stable for days.

Validates the response before writing. A captive portal and a rate-limit
message both arrive with a 200 and would otherwise be drawn as an address.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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