feat(chat): add fullscreen diffs and task change review

[chliny]

Summary

• Add a fullscreen content viewer for code, tool output, and diffs.
• Render colored unified diffs, including apply_patch tool calls.
• Add an end-of-task Review Changes section derived from session messages and patch content.
• Improve diff parsing and display behavior with focused regression tests.

Verification

• npm test: 344 passed, 0 failed.
• npm run typecheck: passed.
• Android release build: assembleRelease -PreactNativeArchitectures=arm64-v8a passed.