fix: resolve welcome template rendering and new tab button duplication bugs

[ThisIs-Developer]

This Pull Request introduces critical production bug fixes to resolve BUG-001 (Welcome Template Missing) and BUG-002 (New Tab Button Duplication) inside the Markdown Viewer application.

Key Changes:

• BUG-001 Resolved (Synchronous Welcome Template): Inlined the full default welcome markdown document into a <script type="text/markdown" id="default-markdown"> element inside index.html. Refactored script.js to retrieve this content synchronously using textContent.trim(), completely eliminating the asynchronous fetch race condition, FOUC, and empty startup pages.
• BUG-002 Resolved (Reused New Tab Button): Refactored renderTabBar() to query .tab-bar for an existing .tab-new-btn element before creating a new one. If present, it is reused, preventing duplication and duplicate listener attachments during tab resets, duplication, renames, and deletions.
• Desktop App Synchronization: Programmatically synchronized all fixes using node prepare.js inside desktop-app/ to ensure complete offline-first parity.

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview, Comment	May 31, 2026 7:13pm

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