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>

internal/dashboard/dto.go

@@ -97,9 +97,11 @@ type clearQueueResponse struct {
 }
 
 type validatorDTO struct {
-	ValidatorID string `json:"validator_id"`
-	OSPortID    string `json:"os_port_id"`
-	State       string `json:"state"`
+	ValidatorID     string     `json:"validator_id"`
+	Hostname        string     `json:"hostname"`
+	OSPortID        string     `json:"os_port_id"`
+	State           string     `json:"state"`
+	LastHeartbeatAt *time.Time `json:"last_heartbeat_at"`
 }
 
 type siteDTO struct {