Retry validator-agent/prober registration; show validator hostname

Both binaries registered with control-api exactly once at startup and
exited (os.Exit(1)) on any failure — including control-api simply not
being up yet (no ordering guarantee between the two at boot/redeploy) or
the admin not having added this validator_id/site_id to the config yet.
Run() now retries registration with capped exponential backoff (3s->30s)
until it succeeds or the process is asked to shut down, instead of
crashing; registerWithRetry is identical in agentcore and probercore
since their Run/register shape already was.

Separately, the admin dashboard's Validators page had no hostname column
even though the agent already reports one on register (mirroring the
prober) and control-api already persists it — only the admin-config read
DTO (validatorDTO in httpapi and dashboard) dropped it before it reached
the template. Added hostname + last_heartbeat_at to that DTO end-to-end
and a Хост/Heartbeat column to validators.html, matching sites.html.

Rebuilt bin/{control-api,admin-dashboard,prober,validator-agent} and
bin/SHA256SUMS per docs/SETUP.md's documented build recipe.

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

bin/SHA256SUMS

@@ -1,4 +1,4 @@
-8f2ad37b7819131a9b25eee9d35a57880c7aa233bbb741f4fd8dc0af33f6241c  control-api
-3288930ce4e09e6793048103e25991f025f71dbedced39dee9bccf821eb72a3d  validator-agent
-586cb533631dfcef2246cdb69e20777f882506755d15afe475399356c1ed2d02  prober
-4729c33a2bce584cba29a8833b0701c0a1cd98bb17bf331aabffc27c4cde940c  admin-dashboard
+dcb137ae6a3f6a59465f2772215d48f98fd303dd1c266645b20ec70194ab7915  control-api
+b7a6068db1d095ae7b73cbd9b7273d6a1629ae017e9c3c9601e103432894a247  validator-agent
+be8e9576c6fbe5e5798b5d4b758a5617b63bea6abe80e326998d10583edbbf5c  prober
+579253ac22d6b9537870cf45aaa85437a4a9dba0c87eab9ba8c49c28e50ee438  admin-dashboard