Broken linkification from inconsistent Markdown processing

Parsing of ```…``` "fence" blocks and `…` inline code in ReplaceQuotedSubstrings does not match that of shared/common-adapters/markdown/shared.tsx, and as a result links are incorrectly detected.

For example:

• ``code` renders as «`code», while ``https://example.com/` renders as «`$>kb$…$<kb$» with a preview box.

• ```\```fenced``` renders as

  \```fenced
  

  while ```\```https://example.com/``` renders as

  `
  ``\
  `
  `$>kb$…$<kb$

  with a preview box.

• ```code``````https://example.com/``` renders as

  code```
  

  https://example.com/
  `
  ``

  (in which https://example.com/ is not a link)

Compare Markdown handling in go/chat/utils/utils.go at ReplaceQuotedSubstrings with shared/common-adapters/markdown/shared.tsx, focusing on fenced and inline code. Reproduce the listed examples and trace where link detection diverges. Done means both paths handle the examples consistently without creating incorrect links or previews.