fix: hide split diff toggle at lg breakpoint

[AbhiVarde]

Problem

Split diff view at the lg breakpoint (1024–1279px) is too cramped, the PR detail page has a resizable conversation panel alongside the diff panel, leaving each code column too narrow to read comfortably.

Fix

Hide the split diff toggle button at the lg breakpoint only. It remains accessible at md (768–1023px) for wider tablet/landscape views and at xl (1280px+) for large desktop screens.

Breakpoint behaviour:

• xs/sm (< 768px) → hidden
• md (768–1023px) → visible
• lg (1024–1279px) → hidden
• xl (1280px+) → visible

Changes

• apps/web/src/components/pr/pr-diff-viewer.tsx - one class change on the split view toggle button

[vercel]

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