Change useSkillFileData to use isEditable instead of isMediaFile:
- Editable files (markdown, code, text) fetch file content for editing
- Non-editable files (image, video, office, unsupported) fetch download URL
This fixes the download button for unsupported files which was incorrectly
using file content (UTF-8 decoded garbage) instead of the presigned URL.
Extract business logic into dedicated hooks to reduce component complexity:
- useFileTypeInfo: file type detection (markdown, code, image, video, etc.)
- useSkillFileData: data fetching with conditional API calls
- useSkillFileSave: save logic with Ctrl+S keyboard shortcut
Also fix Vercel best practice: use ternary instead of && for conditional rendering.
