fix: show save dialog when creating new files (issue 6309)

[su0as]

Description

This PR fixes issue #6309 where the "Create File" command always created files in the workspace root instead of allowing users to choose the location.

Related Issue

Fixes #6309 - Files created in workspace root instead of chosen directory

Changes

• Added showSaveDialog?: boolean property to ApplyToFilePayload interface for backward compatibility
• Updated VS Code extension message handler to show a native save dialog when showSaveDialog flag is set
• Modified frontend to set showSaveDialog: true when creating new files (when fileExists is false)

Testing

Manual Testing Performed

• ✅ Creating new files shows the save dialog
• ✅ Dialog suggests correct filename from relative path (e.g., utils/helper.js suggests helper.js)
• ✅ User can choose any location to save the file
• ✅ Existing files still apply changes without dialog
• ✅ User can cancel file creation
• ✅ Nested directories work correctly (e.g., src/components/Button.tsx)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the style guidelines of this project (Prettier formatted)
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• The fix is focused on the specific issue without unrelated changes
• New and existing unit tests pass locally with my changes
• I have added tests that prove my fix is effective (can be added in follow-up PR)

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Summary by cubic

Fixed an issue where creating a new file always placed it in the workspace root by showing a save dialog so users can choose the file location.

• Bug Fixes
  • Added a save dialog when creating new files, allowing users to pick the directory and filename.
  • Existing files are updated without showing the dialog.

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[su0as]

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