New reverse zone: pick from the networks your records use, sized around them

The Zones page's New reverse zone lists every network the records use without a reverse zone, in every view, also where nothing is suggested (suggestions off, dismissed, public views), marked. Picking one fills in the network and view; size buttons (/48 /56 /64 /128, /16 /24 /32) re-mask it around a record's address, so a /48 becomes the records' own /64. Fill stays on. jarl zone suggest-reverse --all [--view V]; GET /reverse-zones/suggestions?all=1&view=.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/static/assets/names.js

@@ -182,3 +182,17 @@ export function checkDomain(input) {
   if (isReverseZone(name)) notes.push("a reverse zone: choosing \"Reverse zone\" and giving the network is easier");
   return { name, problems, notes };
 }
+
+// maskNetwork returns the network of the given size an address (or a
+// network, "2a02:8013:7f3d:10::/64") is in: maskNetwork("2a02:8013:7f3d:10::5", 48)
+// is "2a02:8013:7f3d::/48". null when it isn't an address.
+export function maskNetwork(input, bits) {
+  const addr = String(input).trim().split("/")[0];
+  const b4 = parseIPv4(addr);
+  const n6 = b4 ? null : parseIPv6(addr);
+  if (!b4 && !n6) return null;
+  const digits = b4 || n6.map((x) => parseInt(x, 16));
+  const step = b4 ? 8 : 4;
+  const net = digits.map((d, i) => { const k = Math.max(0, Math.min(step, bits - i * step)); return d & ~((1 << (step - k)) - 1) & ((1 << step) - 1); });
+  return (b4 ? net.join(".") : sixOf(net.map((d) => d.toString(16)))) + "/" + bits;
+}