feat: GuitarFretboard component

app/app/components/chord-diagram/guitar-fretboard.tsx

@@ -0,0 +1,103 @@
+import type { GuitarVoicing } from '~/lib/chord-voicing';
+
+const FRETS_SHOWN = 4;
+const STRING_COUNT = 6;
+
+interface Props {
+  voicing: GuitarVoicing | null;
+}
+
+export function GuitarFretboard({ voicing }: Props) {
+  if (!voicing) {
+    return (
+      <div className="flex items-center justify-center text-muted-foreground text-xs h-20 w-24">
+        no voicing
+      </div>
+    );
+  }
+
+  const { frets, baseFret, barre } = voicing;
+
+  // Show fret number label when not at open position
+  const showFretLabel = baseFret > 0;
+
+  return (
+    <div className="flex flex-col items-center gap-1">
+      {/* Open/muted string indicators above nut */}
+      <div className="flex" style={{ gap: 2 }}>
+        {frets.map((f, i) => (
+          <div
+            key={i}
+            className="flex items-center justify-center text-xs font-mono"
+            style={{ width: 14 }}
+          >
+            {f === null ? (
+              <span className="text-destructive">✕</span>
+            ) : f === 0 ? (
+              <span className="text-muted-foreground">○</span>
+            ) : null}
+          </div>
+        ))}
+      </div>
+
+      {/* Fretboard grid */}
+      <div className="relative flex" style={{ gap: 0 }}>
+        {/* Fret number label */}
+        {showFretLabel && (
+          <div
+            className="absolute text-xs text-muted-foreground font-mono"
+            style={{ left: -18, top: 0, lineHeight: '18px' }}
+          >
+            {baseFret}
+          </div>
+        )}
+
+        {/* Strings (columns) */}
+        {frets.map((fret, stringIdx) => (
+          <div key={stringIdx} className="relative" style={{ width: 14 }}>
+            {/* Nut or top border */}
+            <div
+              style={{
+                height: showFretLabel ? 2 : 4,
+                background: showFretLabel ? 'hsl(var(--border))' : 'hsl(var(--foreground))',
+                margin: '0 2px',
+              }}
+            />
+            {/* Fret cells */}
+            {Array.from({ length: FRETS_SHOWN }, (_, fretIdx) => {
+              // Open position (baseFret=0): rows represent frets 1-4 (nut is shown above)
+              // Barre position (baseFret>0): rows represent frets baseFret, baseFret+1, …
+              const absoluteFret = baseFret === 0 ? fretIdx + 1 : baseFret + fretIdx;
+              const hasDot = fret !== null && fret > 0 && fret === absoluteFret;
+              return (
+                <div
+                  key={fretIdx}
+                  className="flex items-center justify-center border-b border-border"
+                  style={{ height: 18, borderLeft: stringIdx === 0 ? '1px solid hsl(var(--border))' : undefined, borderRight: '1px solid hsl(var(--border))' }}
+                >
+                  {hasDot && (
+                    <div className="w-3 h-3 rounded-full bg-primary z-10" />
+                  )}
+                </div>
+              );
+            })}
+          </div>
+        ))}
+
+        {/* Barre indicator — only for actual barre chords (not open position) */}
+        {barre !== null && barre > 0 && (
+          <div
+            className="absolute rounded-full bg-primary"
+            style={{
+              left: 2,
+              right: 2,
+              height: 8,
+              top: (showFretLabel ? 2 : 4) + (barre - baseFret) * 18 + 5,
+              zIndex: 5,
+            }}
+          />
+        )}
+      </div>
+    </div>
+  );
+}