Fix the dashboard's pending-changes count; explain zones in the docs

The dashboard counted every open changeset, including empty ones and
other users', so "Pending changes (1 open)" led to an empty Changes page.
It now counts the user's own changesets that contain changes, the status
API's open_changesets skips empty ones, and in review mode a draft created
for an edit the server rejects is discarded instead of left open.

docs/concepts.md: zone types, a new zone's name servers and glue, reverse
zones for any network, and dynamic updates versus API tokens and TSIG keys.

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

internal/web/static/assets/api.js

@@ -116,10 +116,19 @@ export function useDraft(cs) {
 async function ensureDraft() {
   if (state.draft) return state.draft;
   const cs = await api("POST", "/changesets", { body: { description: "" } });
+  cs.fresh = true;
   useDraft(cs);
   return cs;
 }
 
+// dropFreshDraft discards a draft that was created for an edit the server
+// then rejected, so no empty changeset is left open.
+async function dropFreshDraft(cs) {
+  if (!cs.fresh || (state.draft && state.draft.id !== cs.id) || (state.draft && (state.draft.ops || []).length)) return;
+  try { await api("POST", "/changesets/" + cs.id + "/discard"); } catch { /* leave it */ }
+  useDraft(null);
+}
+
 // change sends a configuration change: staged into the draft changeset,
 // or applied at once when immediate mode is on (or opts.immediate).
 // It returns the API response.
@@ -140,6 +149,7 @@ export async function change(method, path, body, opts = {}) {
       useDraft(null);
       return change(method, path, body, opts);
     }
+    await dropFreshDraft(cs);
     throw e;
   }
 }
@@ -152,7 +162,13 @@ export async function changeMany(ops, description, opts = {}) {
     return r;
   }
   const cs = await ensureDraft();
-  const r = await api("POST", "/changesets/" + cs.id + "/ops", { body: { ops } });
+  let r;
+  try {
+    r = await api("POST", "/changesets/" + cs.id + "/ops", { body: { ops } });
+  } catch (e) {
+    await dropFreshDraft(cs);
+    throw e;
+  }
   if (r && r.changeset) state.draft = r.changeset;
   notify();
   return r;