fix(gui): contain sidebar markdown overflow

[JSap0914]

Summary\n- allow the GUI main pane and chat step wrappers to shrink with min-w-0\n- hide horizontal overflow at the chat-scroll container while keeping code blocks horizontally scrollable\n- cap markdown/code blocks to their parent width instead of viewport width\n\nFixes #12910.\n\n## Verification\n- npx prettier --check gui/src/pages/gui/index.tsx gui/src/pages/gui/Chat.tsx gui/src/components/StepContainer/StepContainer.tsx gui/src/components/StyledMarkdownPreview/index.tsx\n- npm --prefix gui run lint -- src/pages/gui/index.tsx src/pages/gui/Chat.tsx src/components/StepContainer/StepContainer.tsx src/components/StyledMarkdownPreview/index.tsx\n- Chromium layout smoke: 420px sidebar with 5000-character unbroken code token kept document scrollWidth at 420px, Enter button right edge at 412px, and overflow inside the code block\n\n## Notes\n- npm --prefix gui run tsc:check was not usable in this checkout after package install because it reports existing core dependency/type errors outside these files.\n- npm --prefix gui test -- src/pages/gui/chat-tests/Chat.test.tsx was not usable because the existing test setup fails before collection with localStorage.getItem is not a function.

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