fix(extract): render failed step errors as plain text, not JSON

[cpcloud]

Summary

• Failed extraction steps (LLM errors, parse failures, timeouts) were rendered through the JSON code-block path (json.Indent + glamour), making error messages hard to read
• Now failed steps skip the JSON path and render with ExtFail style (danger color) as plain text
• Also applies to non-LLM failed steps (e.g. extract failures), which previously used neutral HeaderHint and now use error-colored styling

Reproduction

1. Start an extraction on a document
2. Trigger an LLM failure (e.g. unreachable model, invalid response)
3. Expand the failed LLM step in the extraction overlay
4. Before: error text rendered inside a glamour JSON code block with syntax highlighting
5. After: error text rendered as plain text with danger color

Closes #765

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.19%. Comparing base (f9f0968) to head (4f956e5).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/app/extraction.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
internal/app/extraction.go	76.96% <66.66%> (+0.27%)	⬆️

... and 4 files with indirect coverage changes

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