feat(16-02): enable Share button for viewport URL sharing

- Uncomment shareBtn variable declaration
- Wire Share button click handler to handleShareView()
- Remove design decision TODO - viewport-only sharing confirmed
- Keyboard shortcut Ctrl+Shift+S already functional
- Add ts-ignore comment documenting conditional usage

Author: FlightCore Dev

viewer/src/main.ts

@@ -161,8 +161,8 @@ async function init(): Promise<void> {
   const routingStatus = document.getElementById('routing-status')!;
   const routingProgress = document.getElementById('routing-progress')!;
   const openBtn = document.getElementById('open-btn') as HTMLButtonElement;
-  // TODO: Share feature disabled pending design decision
-  // const shareBtn = document.getElementById('share-btn') as HTMLButtonElement;
+  // @ts-ignore - Used conditionally in web-only block (line 1001-1002)
+  const shareBtn = document.getElementById('share-btn') as HTMLButtonElement;
   const themeToggle = document.getElementById('theme-toggle') as HTMLButtonElement;
   const themeIcon = document.getElementById('theme-icon')!;
   const editorToggleBtn = document.getElementById('editor-toggle') as HTMLButtonElement;
@@ -999,9 +999,8 @@ async function init(): Promise<void> {
 
   // Share button (web only)
   if (!isDesktop()) {
-    // TODO: Share feature needs design decision - share full board state or just viewport?
-    // shareBtn.classList.remove('hidden');
-    // shareBtn.addEventListener('click', handleShareView);
+    shareBtn.classList.remove('hidden');
+    shareBtn.addEventListener('click', handleShareView);
   }
 
   // Keyboard shortcuts