feat(voice): all microphone buttons use the server's speech recognition

- New useVoiceInput hook (lib/voice.ts): "server" records with MediaRecorder
  and sends to /voice/transcribe, "browser" keeps the live recognition,
  "off" or missing browser support hides the microphone.
- Quick dump bar, mobile capture sheet and hold-to-record use it; live
  waves from the real input level (VoiceWave) and "Wird erkannt…".
- Recordings stop after STT_MAX_SECONDS.

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

frontend/src/components/VoiceWave.tsx

@@ -0,0 +1,58 @@
+import React, { useEffect, useRef } from 'react';
+
+interface VoiceWaveProps {
+  /** Input level 0..1; null (unknown, e.g. browser recognition) shows a calm idle wave. */
+  getLevel: () => number | null;
+  bars?: number;
+  /** Bar colour and size come from the caller, e.g. "h-4 text-accent". */
+  className?: string;
+}
+
+// Live wave reacting to the voice. Heights are set directly on the bars every frame, so React does
+// not re-render 60 times a second.
+export const VoiceWave: React.FC<VoiceWaveProps> = ({ getLevel, bars = 5, className = 'h-4 text-accent' }) => {
+  const barRefs = useRef<(HTMLSpanElement | null)[]>([]);
+  const getLevelRef = useRef(getLevel);
+  getLevelRef.current = getLevel;
+
+  useEffect(() => {
+    const history: number[] = new Array(bars).fill(0);
+    let frame = 0;
+    let raf = 0;
+    const reduceMotion = window.matchMedia?.('(prefers-reduced-motion: reduce)').matches;
+    const tick = () => {
+      frame++;
+      const measured = getLevelRef.current();
+      const level = measured ?? 0.25 + 0.2 * Math.sin(frame / 9);
+      // Newest level enters in the middle and travels outwards
+      if (frame % 3 === 0) {
+        history.pop();
+        history.unshift(level);
+      }
+      const mid = Math.floor(bars / 2);
+      barRefs.current.forEach((el, i) => {
+        if (!el) return;
+        const v = reduceMotion ? 0.5 : history[Math.abs(i - mid)] ?? 0;
+        el.style.transform = `scaleY(${Math.max(0.15, Math.min(1, v * 1.4))})`;
+      });
+      raf = requestAnimationFrame(tick);
+    };
+    raf = requestAnimationFrame(tick);
+    return () => cancelAnimationFrame(raf);
+  }, [bars]);
+
+  return (
+    <span className={`inline-flex items-center gap-[3px] ${className}`} aria-hidden="true">
+      {Array.from({ length: bars }, (_, i) => (
+        <span
+          key={i}
+          ref={(el) => {
+            barRefs.current[i] = el;
+          }}
+          className="w-[3px] h-full rounded-full bg-current origin-center transition-transform duration-75"
+          style={{ transform: 'scaleY(0.15)' }}
+        />
+      ))}
+    </span>
+  );
+};