feat(skills): add folder upload support for skill file management

[niiish32x]

Add the ability to upload an entire folder of files to a skill instead of
creating files one by one. This significantly improves the workflow for
users who want to bulk import skill files.

Backend changes:

• Add SkillFileUploadItem, SkillFileBatchUploadRequest, and SkillFileBatchUploadResponse schemas
• Add POST /file/upload_batch endpoint for batch file upload
• Implement batch_upload_files() service method with support for both text and binary files (base64 encoded)

Frontend changes:

• Add batchUploadSkillFiles() API function
• Add "Upload Folder" button to skill detail page
• Support folder selection via webkitdirectory attribute
• Handle text files and binary files appropriately
• Preserve directory structure during upload

This feature addresses the pain point of manually creating skill files
one at a time, making it much easier to import existing skill projects.

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Snapshots:

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Checklist:

• My code follows the style guidelines of this project
• I have already rebased the commits and make the commit message conform to the project standard.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules

[yhjun1026]

LGTM