Expanded diff context

Diff Options can automatically reveal whole-file context for eligible text diffs and can set manual expansion steps to 20, 50, or 100 lines. The preference is local presentation state and never rewrites a patch, staging selection, or working file.

Automatic expansion requires a complete expandable file no larger than 2,000 lines and 512 KiB. Partial reads, binary/large files, and non-expandable diffs stay collapsed. Enabling it affects the visible eligible diff immediately; disabling it does not silently collapse what the user is reviewing, and the explicit collapse action remains available.

Malformed storage falls back to automatic expansion off and a 20-line step. Controls update live in English, playful Hong Kong Cantonese, and bilingual mode. Tests cover bounds, persistence, hunk merging/expansion, immediate updates, collapse, and localization. More detail is in Changed-file trees and diff context.