feat: Add copy-to-clipboard button for version SHA in About modal

Click the clipboard icon next to the version to copy the SHA.
Shows a green checkmark for 2 seconds to confirm the copy.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cjw6k

tidal-harmonics/src/components/ui/BottomLeftButtons.tsx

@@ -8,6 +8,17 @@ export function BottomLeftButtons() {
   const [showAbout, setShowAbout] = useState(false);
   const [showGlossary, setShowGlossary] = useState(false);
   const [showHistory, setShowHistory] = useState(false);
+  const [copied, setCopied] = useState(false);
+
+  const copyVersionToClipboard = async () => {
+    try {
+      await navigator.clipboard.writeText(__GIT_SHA__);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 2000);
+    } catch (err) {
+      console.error('Failed to copy:', err);
+    }
+  };
 
   // Keyboard shortcut to toggle shortcuts help
   useEffect(() => {
@@ -188,7 +199,7 @@ export function BottomLeftButtons() {
                 </button>
               </div>
               <div className="flex items-center gap-3 text-xs text-slate-500">
-                <span>
+                <span className="flex items-center gap-1">
                   Version:{' '}
                   <a
                     href={`https://github.com/cjw6k/tidal-harmonics-visualizer/commit/${__GIT_SHA__}`}
@@ -198,6 +209,21 @@ export function BottomLeftButtons() {
                   >
                     {__GIT_SHA__}
                   </a>
+                  <button
+                    onClick={copyVersionToClipboard}
+                    className="p-0.5 text-slate-600 hover:text-slate-400 transition-colors"
+                    title="Copy SHA to clipboard"
+                  >
+                    {copied ? (
+                      <svg className="w-3.5 h-3.5 text-green-400" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                        <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M5 13l4 4L19 7" />
+                      </svg>
+                    ) : (
+                      <svg className="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                        <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M8 16H6a2 2 0 01-2-2V6a2 2 0 012-2h8a2 2 0 012 2v2m-6 12h8a2 2 0 002-2v-8a2 2 0 00-2-2h-8a2 2 0 00-2 2v8a2 2 0 002 2z" />
+                      </svg>
+                    )}
+                  </button>
                 </span>
                 <span>
                   Made by{' '}