refactor(plugin): rename detection_interval_seconds → detection_interval_minutes (#9366)

Minutes is the natural granularity for detection cadence — every
production handler already set the seconds field to a 60-multiple
(17*60, 30*60, 3600, 24*60*60). Switching to minutes drops the *60
arithmetic and matches the unit conventions used elsewhere in the
plugin worker forms.

- Proto: AdminRuntimeDefaults + AdminRuntimeConfig.detection_interval_*
  field renamed.
- Helpers: durationFromMinutes / minutesFromDuration alongside the
  existing seconds variants in plugin_scheduler.go.
- Handlers: vacuum, ec_balance, balance, erasure_coding, iceberg,
  admin_script, s3_lifecycle now declare DetectionIntervalMinutes.
- Admin: scheduler_status + types + UI templ + plugin_api.go pass
  through the new field; UI label and table cells switch to "min".

weed/admin/dash/plugin_api_test.go

@@ -137,7 +137,7 @@ func TestApplyDescriptorDefaultsToPersistedConfigBackfillsAdminDefaults(t *testi
 			},
 		},
 		AdminRuntimeDefaults: &plugin_pb.AdminRuntimeDefaults{
-			DetectionIntervalSeconds: 60,
+			DetectionIntervalMinutes: 60,
 			DetectionTimeoutSeconds:  300,
 		},
 	}
@@ -152,7 +152,7 @@ func TestApplyDescriptorDefaultsToPersistedConfigBackfillsAdminDefaults(t *testi
 	if !ok || scriptKind.StringValue == "" {
 		t.Fatalf("expected non-empty script default, got=%+v", script)
 	}
-	if config.AdminRuntime.DetectionIntervalSeconds != 60 {
+	if config.AdminRuntime.DetectionIntervalMinutes != 60 {
 		t.Fatalf("expected runtime detection interval default to be backfilled")
 	}
 }