client: fix reset-error handling, undersized qty buttons, chart resize,

dedupe j()/errText(), stale-fetch guard, empty bar-picker state

Six small, mostly-unrelated items from the #47 review round (#57):

1. 'Statistik zurücksetzen' now checks res.ok and shows the server's
   error instead of silently re-rendering the un-reset data on a 401/500
   with no indication anything went wrong.
2. DayChart now tracks its container width via ResizeObserver and calls
   uPlot's setSize() on change — previously read once at plot-creation
   time, so a resize/orientation-change (more visible on the phone/
   tablet path into /stats) left the chart the wrong width.
3. .qty-btn (cart +/- buttons) grown from 28x28px to 44x44px, the
   conventional minimum touch target — was the one conspicuously small
   interactive control in the money path, at a fast-moving bar counter.
4. Tablet's bar-config fetch (App.tsx) now guards against an
   out-of-order resolution if barId changes again while a previous
   api.config() call is still in flight. Currently unreachable (nothing
   triggers a second barId change mid-fetch today) but closes the gap
   before the next 'add a cancel button to the loading screen' change
   could make it live.
5. j()/errText() were duplicated byte-for-byte between Admin.tsx and
   Stats.tsx (Admin.tsx's own comment noted this exact duplication had
   already caused drift once before) — centralized in api.ts, both files
   now import instead of redeclaring.
6. Tablet's bar picker now shows an explicit empty-state message when
   zero bars exist (fresh install, or every bar deleted), previously
   indistinguishable from 'still loading'.

Closes #57. Verified: tsc --noEmit and vite build both clean.

client/src/tablet/App.tsx

@@ -24,7 +24,17 @@ export function App() {
   useEffect(() => {
     if (barId == null) return;
     setConfig(null);
-    api.config(barId).then(setConfig).catch(e => setError(String(e)));
+    // Guards against an out-of-order resolution if barId changes again
+    // while this fetch is still in flight (e.g. a future "cancel loading"
+    // control) — currently unreachable since nothing triggers a second
+    // barId change mid-fetch today, but cheap to close now rather than
+    // leave as a defensive gap the next loading-screen change could make
+    // live.
+    let cancelled = false;
+    api.config(barId)
+      .then(c => { if (!cancelled) setConfig(c); })
+      .catch(e => { if (!cancelled) setError(String(e)); });
+    return () => { cancelled = true; };
   }, [barId]);
 
   function pick(id: number) {

client/src/tablet/BarPicker.tsx

@@ -21,6 +21,10 @@ export function BarPicker({ bars, onPick }: Props) {
       <h1>Bar auswählen</h1>
       {bars == null ? (
         <p>Lade…</p>
+      ) : bars.length === 0 ? (
+        // A fresh install (or every bar deleted) otherwise renders nothing
+        // but the header here — indistinguishable from "still loading".
+        <p class="muted">Keine Bars angelegt — bitte im Backoffice anlegen.</p>
       ) : (
         bars.map(b => (
           <button key={b.id} onClick={() => onPick(b.id)}>