Fix captioned Markdown image rendering

[grapeot]

Summary

• Normalize standalone Markdown image lines before rendering when the next line contains caption/text.
• Apply the normalizer in File Preview, Chat message rendering, and Reasoning cards.
• Keep true inline images unchanged.
• Add unit tests covering caption separation, already-separated image blocks, and true inline images.
• Update docs/working.md.

Root cause

When Markdown is written as:

![image](url)
*caption*

with no blank line, Markdown renderers can treat the image as an inline image in the same paragraph. That bypasses the block-image rendering path and can produce incorrect sizing.

Test plan

• ./gradlew testDebugUnitTest