feat: add -videoID flag to set thumbnail on existing videos

[dandaka]

Summary

• Adds a new -videoID flag that, combined with -thumbnail, allows setting a thumbnail on an already-uploaded video via the YouTube Data API v3 thumbnails.set endpoint
• Solves the real-world problem where upload succeeds but thumbnail fails (e.g., file too large) — currently requires manual fix in YouTube Studio
• -videoID + -thumbnail triggers the new thumbnail-only mode; -videoID + -filename is rejected; existing upload behavior is unchanged

Changes

• files.go: Added VideoID field to Config struct
• cmd/youtubeuploader/main.go: Added -videoID flag, validation logic to route between upload and thumbnail-set modes
• thumbnail.go: New file with SetThumbnail() function — opens thumbnail (file or URL via existing Open()), builds OAuth client, calls Thumbnails.Set()

Usage

# Set thumbnail on existing video
youtubeuploader -videoID LSfZhFqSH0c -thumbnail cover.jpg

# Still works as before
youtubeuploader -filename video.mp4 -thumbnail cover.jpg -title "My Video"

Test plan

• go build compiles cleanly
• -videoID without -thumbnail → error
• -videoID with -filename → error
• -videoID with -thumbnail → successfully sets thumbnail via YouTube API (tested against a real video)