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/config/tools.ts

@@ -67,6 +67,7 @@ export const categories = [
         tools: [
             { id: 'ocr-pdf', name: 'OCR PDF', icon: 'scan-text', subtitle: 'Make a PDF searchable and copyable.' },
             { id: 'merge', name: 'Merge PDF', icon: 'combine', subtitle: 'Combine multiple PDFs into one file.' },
+            { id: 'alternate-merge', name: 'Alternate & Mix Pages', icon: 'shuffle', subtitle: 'Combine PDFs by alternating pages from each.' },
             { id: 'organize', name: 'Organize PDF', icon: 'grip', subtitle: 'Reorder pages by dragging and dropping.' },
             { id: 'duplicate-organize', name: 'Duplicate & Organize', icon: 'files', subtitle: 'Duplicate, reorder, and delete pages.' },
             { id: 'split', name: 'Split PDF', icon: 'scissors', subtitle: 'Extract a range of pages into a new PDF.' },