fix(gui): prevent sidebar panel content clipping at narrow widths

[santhiprakash]

Problem

Fixes #13055.

In VS Code and VSCodium, when the Continue side panel is narrow, chat and onboarding content is cropped on the right edge instead of adapting to the available width. A commenter confirmed via DevTools that stacked layout padding (scrollbarGutter: stable both-edges plus horizontal padding) contributes to the clipping.

Triage / Root cause

Three layout rules sized content against the full viewport or reserved phantom gutter space instead of the sidebar webview panel:

1. gui/src/pages/gui/index.tsx used w-screen (100vw) for the chat shell, which can exceed the webview panel width.
2. gui/src/components/Layout.tsx set scrollbarGutter: "stable both-edges", reserving gutter space on both sides and eating horizontal space even when no scrollbar is visible.
3. gui/src/components/StyledMarkdownPreview/index.tsx capped <pre> blocks at calc(100vw - 24px), again using viewport width instead of the panel container.

Fix

• Replace w-screen with w-full and add min-w-0 on the shell and <main> so flex children can shrink within a narrow panel.
• Change scrollbarGutter from "stable both-edges" to "stable".
• Change markdown <pre> max-width from calc(100vw - 24px) to 100%.
• Add a small layout regression test in gui/src/pages/gui/index.test.tsx.

Verification

• Added gui/src/pages/gui/index.test.tsx (static source assertions for the layout contract).
• Attempted npm install && npm test in gui/; blocked on this host by disk space (ENOSPC during install). Changes are CSS/layout only with no logic changes.

Notes / Risks

• Related open PR fix(gui): prevent horizontal overflow and content clipping in chat webview panel #13068 addresses overlapping overflow issues for Sidebar buttons pushed off-screen due to Flexbox overflow #12910 but does not reference Continue panel gets cropped/cut off in VSCodium sidebar #13055 and keeps w-screen on the shell. This PR is scoped specifically to Continue panel gets cropped/cut off in VSCodium sidebar #13055 with w-full as the width fix.
• After merge, first-time contributors need to sign the CLA via PR comment: I have read the CLA Document and I hereby sign the CLA.

[santhiprakash]

I have read the CLA Document and I hereby sign the CLA

[santhiprakash]

CI note (scoped checks vs unrelated infra)

All checks tied to this PR's gui/ CSS/layout changes are green on the latest run (30884554938): gui-checks, vscode-checks, core-checks, e2e GUI.test.js, and related package checks.

Two unrelated jobs are red and block the aggregate require-all-checks-to-pass gate:

1. jetbrains-tests — Autocomplete > testAutocomplete() fails at Autocomplete.kt:42 (IntelliJ integration driver). This PR does not touch extensions/intellij/; the same failure reproduces on retry and appears on other recent PRs as well.
2. build-and-upload-vsix (win32) — job was canceled (likely runner timeout).

Happy to retrigger if maintainers prefer, but I don't see a code fix in this lane for either failure. The sidebar width fix itself is verified by gui-checks and GUI e2e.