fix(viewer): stack suggested code panels

[Linxiushen]

Description

Display the existing-code and suggested-change panels as a single vertical stack at every viewport size. Each snippet now gets the full comment width, making longer lines easier to compare and removing the redundant mobile-only grid override.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• Focused Viewer template/static tests
• go vet ./...
• Manual browser testing at desktop and mobile widths

At 1265px, both panels render at the same 1026px width on separate rows. At a 390px viewport, they remain stacked within the comment card without panel overflow.

go test ./internal/viewer also reaches three existing permission-mode assertions that are not portable to Windows (TestHandleRepos_PermissionDenied, TestDiscoverRepos_SkipsUnreadableSubdir, and TestListSessions_SkipsUnreadableFiles). The focused Viewer rendering tests pass.

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• Relevant tests pass locally with my changes
• Documentation is unchanged because the Viewer behavior is self-evident
• I have signed the CLA

Related Issues

Closes #733

[CLAassistant]

All committers have signed the CLA.

[github-actions]

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