feat(docs): support local image upload in docs +create

[SoIomon]

Summary

When using docs +create --markdown, local image references like ![alt](path/to/image.jpg) fail because the server tries to download them as URLs. This PR adds client-side pre-processing to detect, upload, and replace local image paths before sending markdown to the server.

Changes

• New file: docs_create_images.go — Core logic:

  • Regex-based markdown image detection (![alt](path))
  • Distinguishes local paths from URLs
  • Two-phase document creation: create empty doc → upload images → update with full markdown
  • Security: path traversal prevention, format/size validation, SafeInputPath check

• New file: docs_create_images_test.go — Unit tests for image detection, path validation, and markdown replacement

• Modified: docs_create.go — Adds --base-dir flag and integrates two-phase creation flow

New flag: --base-dir

Specifies the base directory for resolving relative image paths (defaults to CWD):

lark-cli docs +create --markdown "$(cat doc.md)" --base-dir /path/to/doc/dir

Workflow (when local images detected)

1. Parse markdown for ![alt](local/path) references
2. Create document with title only
3. For each local image: create empty image block → upload via drive API → get file_token
4. Replace local paths with file_tokens in markdown
5. Update document content with mode=overwrite

Security

• Rejects absolute paths (must be relative to --base-dir)
• Prevents path traversal (../ cannot escape base-dir)
• Only allows common image formats (jpg/jpeg/png/gif/bmp/webp)
• 20MB file size limit
• Invalid images produce warnings but don't block document creation

Note: This PR depends on #56 (table auto-width) which modifies the same file.

Test plan

• go build ./... passes
• go test ./shortcuts/doc/... passes
• Manual test: create document with local images using --base-dir

[SoIomon]

图片上传方案有设计缺陷，修复后重新提交