fix: prevent Enter/send button from being pushed off-screen by long code blocks (#12958)

[AmariahAK]

Description

When AI responses contain long code blocks without line breaks, the chat sidebar stretches horizontally, pushing the Enter/send button out of view. This fix addresses the root cause in two places:
Root causes:

1. StyledMarkdownPreview/index.tsx: <pre> elements used max-width: calc(100vw - 24px). In a VS Code webview sidebar, 100vw can be the full VS Code window width, not the actual sidebar width, allowing code blocks to grow far beyond the panel.
2. Chat.tsx StepsDiv: The scrollable history container (flex-1) lacked min-w-0. CSS flexbox defaults to min-width: auto for flex children, preventing the container from shrinking below oversized content, shoving the input area + Enter button off-screen.
   Changes (4 files, targeted and surgical):

• gui/src/pages/gui/Chat.tsx — added min-w-0 to StepsDiv so the flex container clips overflow instead of growing
• gui/src/components/StyledMarkdownPreview/index.tsx — changed pre max-width from calc(100vw - 24px) to 100%, and overflow-x from scroll to auto
• gui/src/components/StyledMarkdownPreview/SyntaxHighlightedPre.tsx — added max-width: 100% to StyledPre
• gui/src/components/StepContainer/StepContainer.tsx — added min-w-0 to the message wrapper div
  Unlike the temporary CSS hotfix in Sidebar buttons pushed off-screen due to Flexbox overflow #12910 — which applies min-width: 0 to all flex elements globally and word-break: break-all to all <pre>/<code> elements (making code unreadable) — this fix is targeted at only the specific elements in the overflow chain. No wildcards, no !important, no code readability sacrifice.
  Closes Enter / send message button goes out of the screen over time #12958
  Closes Continue chat pane sizing error #12912

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created (N/A — CSS-only layout fix)
• The relevant tests, if any, have been updated or created (N/A — CSS-only layout fix; visual verification appropriate)

Screen recording or screenshot

N/A — this is a CSS/layout fix with no visual changes to the UI; it prevents the layout from breaking. To verify, test with a prompt that generates a long code block (e.g., "Write a Python script with a 500+ character string on one line") and confirm the Enter button remains visible.

Tests

No new tests added — this is a CSS flexbox layout fix targeting four specific elements. The existing test suites pass (lint-staged + Prettier verified on commit). Manual verification procedure:

1. Launch extension in debug mode (F5 from VS Code)
2. Send a prompt that produces a long code block without line breaks
3. Confirm the Enter/send button stays visible and the sidebar does not stretch horizontally
4. Confirm code blocks still scroll horizontally within their container for long lines
5. Test in both sidebar panel and bottom panel positions

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[AmariahAK]

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