refactor(tui): use native fenced-code highlighting

[TheGreatAxios]

Summary

• route fenced-code colour through OpenTUI’s native tree-sitter renderer while keeping the synchronous parser geometry plain
• pin settled and streaming assistant fences to the production transcript syntax style, with unsupported Python remaining visible and plain
• own fetch and RequestInfo types at the root TypeScript boundary and keep blank fenced lines to gutter-only geometry
• remove highlight.js and reduce the bundled dist artifact by 313,352 bytes and the compiled binary by 180,576 bytes

Verification

• bun test ./src/tui/syntax-highlight.test.ts ./src/tui/markdown-parser.test.ts passes (79 tests)
• removing markdownBodyOptions().syntaxStyle makes only the settled and streaming production transcript keeper cases fail; restoring it makes all 10 highlighting tests pass
• bun run typecheck passes with DOM in tsconfig.base.json and no leaf DOM triple-reference
• bun run check passes (8,596 tests across 551 files)
• bun run build:bin passes
• a temporary production-seam smoke compiled with bun build --compile --minify --external '@opentui/core-*', executed as a native artifact, and exited 0 with javascriptHighlighted=true, pythonVisible=true, and pythonHighlighted=false; its source and binary are removed after the run

Fixes CL-9009

[linear-code]

CL-9009