admin: surface silent failures, guard against double-submit on add

- Drinks.add()/patch() now check res.ok and alert() the error text,
  matching the pattern every other mutator in this file already
  used (Drinks.del(), everything in Bars) — previously a rejected
  name/price change just re-fetched the (unchanged) list with no
  indication anything went wrong.
- Stats.reload(), Drinks.reload(), Bars.reload() now .catch() a
  failed fetch/non-ok response instead of leaving an unhandled
  promise rejection — a network hiccup used to leave the section
  stuck on 'Lade…' forever with no visible error.
- Drinks 'Hinzufügen' and Bars 'Bar hinzufügen' are now disabled
  while their POST is in flight, mirroring the submitting guard
  Sale.tsx already uses — a fast double-tap could otherwise fire two
  requests before the first reload() resolved.

client/src/admin/Admin.tsx

@@ -93,7 +93,9 @@ function fmtDay(day: string): string {
 
 function Stats() {
   const [data, setData] = useState<{ totals: Totals[]; per_drink: PerDrink[]; by_day: ByDay[] } | null>(null);
-  function reload() { fetch('/admin/api/stats').then(j).then(setData); }
+  function reload() {
+    fetch('/admin/api/stats').then(j).then(setData).catch(e => alert(`Fehler beim Laden: ${e}`));
+  }
   useEffect(reload, []);
 
   async function reset() {
@@ -171,27 +173,37 @@ function Drinks() {
   const [list, setList] = useState<Drink[]>([]);
   const [name, setName] = useState('');
   const [price, setPrice] = useState('');
+  const [adding, setAdding] = useState(false);
 
-  function reload() { fetch('/admin/api/drinks').then(j).then(setList); }
+  function reload() {
+    fetch('/admin/api/drinks').then(j).then(setList).catch(e => alert(`Fehler beim Laden: ${e}`));
+  }
   useEffect(reload, []);
 
   async function add() {
     const cents = Math.round(parseFloat(price) * 100);
-    if (!name || !Number.isFinite(cents)) return;
-    await fetch('/admin/api/drinks', {
-      method: 'POST',
-      headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({ name, price_cents: cents }),
-    });
-    setName(''); setPrice(''); reload();
+    if (!name || !Number.isFinite(cents) || adding) return;
+    setAdding(true);
+    try {
+      const res = await fetch('/admin/api/drinks', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ name, price_cents: cents }),
+      });
+      if (!res.ok) { alert(`Fehler: ${await errText(res)}`); return; }
+      setName(''); setPrice(''); reload();
+    } finally {
+      setAdding(false);
+    }
   }
 
   async function patch(id: number, body: Partial<Drink>) {
-    await fetch(`/admin/api/drinks/${id}`, {
+    const res = await fetch(`/admin/api/drinks/${id}`, {
       method: 'PATCH',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify(body),
     });
+    if (!res.ok) { alert(`Fehler: ${await errText(res)}`); return; }
     reload();
   }
 
@@ -237,7 +249,7 @@ function Drinks() {
       <div class="row">
         <input placeholder="Name" value={name} onInput={(e: any) => setName(e.currentTarget.value)} />
         <input placeholder="Preis €" type="number" step="0.01" value={price} onInput={(e: any) => setPrice(e.currentTarget.value)} />
-        <button onClick={add}>Hinzufügen</button>
+        <button onClick={add} disabled={adding}>Hinzufügen</button>
       </div>
     </>
   );
@@ -333,10 +345,11 @@ function Bars() {
   const [bars, setBars] = useState<BarRow[]>([]);
   const [drinks, setDrinks] = useState<Drink[]>([]);
   const [newName, setNewName] = useState('');
+  const [addingBar, setAddingBar] = useState(false);
 
   function reload() {
-    fetch('/admin/api/bars').then(j).then(setBars);
-    fetch('/admin/api/drinks').then(j).then(setDrinks);
+    fetch('/admin/api/bars').then(j).then(setBars).catch(e => alert(`Fehler beim Laden: ${e}`));
+    fetch('/admin/api/drinks').then(j).then(setDrinks).catch(e => alert(`Fehler beim Laden: ${e}`));
   }
   useEffect(reload, []);
 
@@ -359,18 +372,23 @@ function Bars() {
 
   async function addBar() {
     const name = newName.trim();
-    if (!name) return;
-    const res = await fetch('/admin/api/bars', {
-      method: 'POST',
-      headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({ name, pfand_cents: 200 }),
-    });
-    if (!res.ok) {
-      alert(`Fehler: ${await errText(res)}`);
-      return;
+    if (!name || addingBar) return;
+    setAddingBar(true);
+    try {
+      const res = await fetch('/admin/api/bars', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ name, pfand_cents: 200 }),
+      });
+      if (!res.ok) {
+        alert(`Fehler: ${await errText(res)}`);
+        return;
+      }
+      setNewName('');
+      reload();
+    } finally {
+      setAddingBar(false);
     }
-    setNewName('');
-    reload();
   }
 
   return (
@@ -412,7 +430,7 @@ function Bars() {
           value={newName}
           onInput={(e: any) => setNewName(e.currentTarget.value)}
         />
-        <button onClick={addBar}>Bar hinzufügen</button>
+        <button onClick={addBar} disabled={addingBar}>Bar hinzufügen</button>
       </div>
     </>
   );