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/stats/DayChart.tsx

@@ -61,5 +61,21 @@ export function DayChart({ labels, series }: { labels: string[]; series: ChartSe
     return () => { plotRef.current?.destroy(); plotRef.current = null; };
   }, [labels, series]);
 
+  // Width was only ever read once at plot-creation time — no resize/
+  // orientation-change handling, minor on desktop but more visible on the
+  // phone/tablet path into /stats. setSize() is uPlot's cheap resize path
+  // (no destroy/rebuild), separate from the effect above since it only
+  // needs to run once for the holder's lifetime, not on every data change.
+  useEffect(() => {
+    const holder = holderRef.current;
+    if (!holder || typeof ResizeObserver === 'undefined') return;
+    const ro = new ResizeObserver(entries => {
+      const width = entries[0]?.contentRect.width;
+      if (width && plotRef.current) plotRef.current.setSize({ width, height: 220 });
+    });
+    ro.observe(holder);
+    return () => ro.disconnect();
+  }, []);
+
   return <div ref={holderRef} class="chart" />;
 }