Health: "Last apply" reads "Kernel in sync"

The health row that shows when the config was last written to the
kernel is now called Kernel in sync, with the time since then, or Out
of sync and the kernel's error when applying failed. The API keeps the
check's name, so clients are unaffected.

app.js

@@ -1856,7 +1856,8 @@
           if (/^table /.test(c.detail)) { t.raw = c.detail.replace(/ (present|missing)$/, ''); t.problem = null; }
           break;
         case 'Last apply':
-          if (c.ok) { const iso = c.detail.replace(/^applied /, ''); t.status = ago(iso); t.title = fmtStamp(iso); } else t.status = 'Failed';
+          t.label = 'Kernel in sync';
+          if (c.ok) { const iso = c.detail.replace(/^applied /, ''); t.status = ago(iso); t.title = fmtStamp(iso); } else t.status = 'Out of sync';
           break;
         case 'Latency check': t.status = c.ok ? 'Tunnel ping works' : 'Failing'; break;
       }