feat(pdf-tools): add alternate merge tool for mixing pdf pages

Implement new tool that allows users to combine PDFs by alternating pages from each document. Includes UI components, logic for processing, and test coverage.

- Add new tool to configuration arrays and categories
- Create UI with drag-and-drop file ordering
- Implement core logic for alternating pages
- Add comprehensive unit tests

src/js/ui.ts

@@ -1819,4 +1819,22 @@ posterize: () => `
     </div>
 `,
 
+'alternate-merge': () => `
+    <h2 class="text-2xl font-bold text-white mb-4">Alternate & Mix Pages</h2>
+    <p class="mb-6 text-gray-400">Combine pages from 2 or more documents, alternating between them. Drag the files to set the mixing order (e.g., Page 1 from Doc A, Page 1 from Doc B, Page 2 from Doc A, Page 2 from Doc B, etc.).</p>
+    ${createFileInputHTML({ multiple: true, accept: 'application/pdf', showControls: true })}
+    
+    <div id="alternate-merge-options" class="hidden mt-6">
+        <div class="p-3 bg-gray-900 rounded-lg border border-gray-700 mb-3">
+            <p class="text-sm text-gray-300"><strong class="text-white">How it works:</strong></p>
+            <ul class="list-disc list-inside text-xs text-gray-400 mt-1 space-y-1">
+                <li>The tool will take one page from each document in the order you specify below, then repeat for the next page until all pages are used.</li>
+                <li>If a document runs out of pages, it will be skipped, and the tool will continue alternating with the remaining documents.</li>
+            </ul>
+        </div>
+        <ul id="alternate-file-list" class="space-y-2"></ul>
+        <button id="process-btn" class="btn-gradient w-full mt-6" disabled>Alternate & Mix PDFs</button>
+    </div>
+`,
+
 };