Add support for opening external links in system browser

[ptheofan]

Summary

This PR adds functionality to handle external links (http, https, mailto) in the markdown viewer by opening them in the system's default browser or handler, rather than attempting to navigate within the application.

Key Changes

• MarkdownViewer: Added setupLinkHandling() to intercept clicks on external links and open them via the system browser
• MarkdownViewer: Added isExternalUrl() helper to identify external URLs by protocol (http, https, mailto)
• MarkdownViewer: Enhanced context menu to show "Copy Link" and "Open in Default Browser" options for external links
• ShellHandler: Implemented new OPEN_EXTERNAL IPC handler with protocol validation to prevent opening dangerous URLs (file://, javascript://, etc.)
• API Types: Added openExternal() method to ShellAPI interface and corresponding IPC channel
• Preload: Exposed shell.openExternal() to renderer process via electron API
• Tests: Added comprehensive test coverage for the new OPEN_EXTERNAL handler, including validation of allowed protocols and rejection of invalid URLs

Implementation Details

• External link detection uses URL parsing with try-catch to safely validate URLs
• Protocol whitelist (http:, https:, mailto:) is enforced at the main process level for security
• Context menu for links provides both copy and open actions for user convenience
• All external link operations are properly prevented from default browser navigation behavior

https://claude.ai/code/session_01A83A3GHjE9aQz4C6zhwar9