fix(captures+chord): Stop button stops, ChordPicker accepts shorter chords

Two unrelated correctness bugs caught in PR review:

- The Play As "Stop" button was wired to handlePlayAs() unconditionally, so clicking it during playback kicked a fresh generation instead of halting. Now pauses the player when the click came from the main button while playbackState is 'playing'. Picking a different voice from the dropdown still kicks a new generation as before.
- ChordPicker tracked the peak set of held keys but seeded the peak from initialKeys, so a user who opened the picker with a 3-key chord saved couldn't replace it with a 2-key chord — the candidate length never beat the seed. The peak now resets on the first press of a fresh sequence (when no keys were held immediately prior), then grows monotonically within that hold.

app/src/components/CapturesTab/CapturesTab.tsx

@@ -154,6 +154,8 @@ export function CapturesTab() {
   const playerIsPlaying = usePlayerStore((s) => s.isPlaying);
   const isPlayerVisible = !!audioUrl;
 
+  const setIsPlaying = usePlayerStore((s) => s.setIsPlaying);
+
   const addPendingGeneration = useGenerationStore((s) => s.addPendingGeneration);
   const pendingGenerationIds = useGenerationStore((s) => s.pendingGenerationIds);
 
@@ -410,6 +412,12 @@ export function CapturesTab() {
 
   const handlePlayAs = (voice?: VoiceProfileResponse) => {
     if (!selected) return;
+    // Stop the current playback when the button is in its 'playing' state
+    // and the user clicked the main button without picking a new voice.
+    if (!voice && playbackState === 'playing') {
+      setIsPlaying(false);
+      return;
+    }
     const target = voice ?? playAsVoice;
     if (!target) {
       toast({

app/src/components/ChordPicker/ChordPicker.tsx

@@ -95,11 +95,12 @@ export function ChordPicker({
         if (prev.has(canonical)) return prev;
         const next = new Set(prev);
         next.add(canonical);
-        // Update peak whenever the live set grows. Comparing against
-        // the captured chord (which may be the previous saved value)
-        // would lose the user's first new keypress.
         setCaptured((prevCaptured) => {
           const candidate = sortChordKeys(Array.from(next));
+          // First key in a fresh sequence replaces the peak — otherwise a
+          // user trying to swap a longer saved chord for a shorter one is
+          // stuck because their candidate never beats the seed length.
+          if (prev.size === 0) return candidate;
           return candidate.length >= prevCaptured.length ? candidate : prevCaptured;
         });
         return next;