Add “Copy All Relative File Paths to Clipboard” to commit and file context menus

[TERAKITERAKICHI]

Summary

This PR adds a new context menu action to copy all changed file paths (relative paths) in one step.

Added

• Copy All Relative File Paths to Clipboard in the Commit Details file list context menu.
• Copy All Relative File Paths to Clipboard in the commit row context menu.

Behavior

• Collects all newFilePath values from the selected commit’s file changes.
• Joins them with newline separators.
• Uses the existing copyToClipboard message flow to write to clipboard.

Implementation Notes

• Front-end only change in main.ts.
• Reused existing clipboard command (copyToClipboard) and avoided backend changes.
• Introduced shared helper methods for:
  • collecting/copying all relative paths
  • deciding menu visibility for commit-row context menu

Validation

• Built successfully with:
  • npm run compile-web
• Manually verified:
  • menu item appears in Commit Details file context menu
  • menu item appears in commit row context menu
  • clipboard contains newline-separated relative file paths as expected

Notes

• The copied paths are relative paths (newFilePath), including renamed files as their new path.
• Deleted files are included if present in the commit file changes.