backup tile: unknown size shows a dash, not 0 B (R-517 follow-up, measured on 9201)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/web/backup_handlers.go

@@ -114,6 +114,10 @@ type guestBackupView struct {
 
 	CanTrigger bool // a backup trigger (quiesce loop) is wired
 
+	// SizeUnknown (R-517): the shown success was read back from storage after an agent restart, so
+	// its size is not known. The tile prints „–", never „0 B" (measured live on 9201, 2026-09-15).
+	SizeUnknown bool
+
 	// Tiers (R-517, agent >= v0.131.0) is the per-tier truth. Empty on an older agent — the tile then
 	// renders the single latest record as before.
 	Tiers []guestTierView
@@ -182,6 +186,7 @@ func buildTierViews(v *guestBackupView, tiers []agentapi.TierBackupState, cadenc
 		if tv.HasSuccess && (t.Primary || !v.HasBackup) && (t.Primary || tv.SuccessAt.After(newestOK)) {
 			v.HasBackup, v.Success = true, true
 			v.StartedAt, v.SizeBytes, v.Target = tv.SuccessAt, tv.SizeBytes, tv.Label
+			v.SizeUnknown = !tv.SizeKnown
 			newestOK = tv.SuccessAt
 		}
 		v.Tiers = append(v.Tiers, tv)