Desktop app renders edit tool diffs as full-file rewrites instead of changed hunks

Summary

In the desktop app, every edit tool call is rendered in the conversation as a
full-file before/after diff (+184 -184 for a 184-line file), even when the edit
only touched one line. In the CLI/TUI the same edit shows only the changed hunk.
This makes large files unreadable — a single-line change produces a diff covering the
entire file.

Repro (real case from our project)

• File: records/views/home.py (184 lines)
• The agent changed exactly 1 line (line 34) via the edit tool:

@@ -31,7 +31,7 @@
 def wework_verify(request):
     """企业微信域名验证文件"""
-    file_path = Path(__file__).resolve().parent.parent / "verify" / "WW_verify_JjNNFXzblKY8zlPe.txt"
+    file_path = Path(__file__).resolve().parent.parent.parent / "verify" / "WW_verify_JjNNFXzblKY8zlPe.txt"

Verified with git diff HEAD~1 -- records/views/home.py: this is the only hunk.

Desktop app (observed)

The tool result is displayed as a full-file rewrite: +184 -184 (all 184 lines
before/after), forcing the reader to visually hunt for the changed line.

CLI/TUI (expected behavior)

The same edit is displayed as just the changed hunk — only the affected line's
old/new content.

Expected behavior

Desktop app should render edit tool results the same way the TUI does: a hunk-level
diff showing only the lines that actually changed (the oldString/newString
content), not the entire snapshotted file.

Impact

• Minor single-line changes are indistinguishable from full rewrites.
• Reviewing diffs in the desktop app for large files (500+ lines) is impractical;
  users must switch to VSCode/CLI to see what actually changed.
• It surfaces the wrong "changed-lines" count (+184 -184) in the UI.

Environment

• opencode version: 1.18.18
• Platform: Windows (win32), desktop app GUI
• Same project, same edit, same agent: CLI/TUI renders correctly; desktop does not.

Suggestion

Reuse the same diff computation the TUI uses for edit tool results (based on the
tool's oldString/newString), or clamp the desktop diff view to changed hunks rather
than snapshot-to-snapshot file diffing.