feat(patch): Add patch application and backup functions

- Add create_backup() to backup files before patching
- Add apply_patch_to_file() to apply unified diff
- Add restore_from_backup() for rollback
- Add cleanup_backup() to remove backup files
- Add 15 unit tests for all patch operations

tests/test_diff_viewer.py

@@ -1,5 +1,12 @@
 import pytest
-from src.diff_viewer import parse_diff, DiffFile, DiffHunk, parse_hunk_header, get_line_color, render_diff_text_immediate
+import tempfile
+import os
+from pathlib import Path
+from src.diff_viewer import (
+  parse_diff, DiffFile, DiffHunk, parse_hunk_header, 
+  get_line_color, render_diff_text_immediate,
+  create_backup, apply_patch_to_file, restore_from_backup, cleanup_backup
+)
 
 def test_parse_diff_empty() -> None:
   result = parse_diff("")
@@ -101,4 +108,74 @@ def test_render_diff_text_immediate() -> None:
   assert ("File: test.py", "white") in output
   assert ("@@ -1 +1 @@", "cyan") in output
   assert ("-old", "red") in output
-  assert ("+new", "green") in output
+  assert ("+new", "green") in output
+
+def test_create_backup() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("original content\n")
+    
+    backup_path = create_backup(str(test_file))
+    assert backup_path is not None
+    assert Path(backup_path).exists()
+    assert Path(backup_path).read_text() == "original content\n"
+
+def test_create_backup_nonexistent() -> None:
+  result = create_backup("/nonexistent/file.py")
+  assert result is None
+
+def test_apply_patch_simple() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("old\n")
+    
+    patch = f"""--- a/{test_file.name}
++++ b/{test_file.name}
+@@ -1 +1 @@
+-old
++new"""
+    
+    success, msg = apply_patch_to_file(patch, tmpdir)
+    assert success
+    assert test_file.read_text() == "new\n"
+
+def test_apply_patch_with_context() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "example.py"
+    test_file.write_text("line 1\nline 2\nline 3\n")
+    
+    patch = f"""--- a/{test_file.name}
++++ b/{test_file.name}
+@@ -1,3 +1,3 @@
+-line 1
+-line 2
++line one
++line two
+ line 3"""
+    
+    success, msg = apply_patch_to_file(patch, tmpdir)
+    assert success
+    content = test_file.read_text()
+    assert "line one" in content
+    assert "line two" in content
+
+def test_restore_from_backup() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("modified\n")
+    backup_file = test_file.with_suffix(".py.backup")
+    backup_file.write_text("original\n")
+    
+    success = restore_from_backup(str(test_file))
+    assert success
+    assert test_file.read_text() == "original\n"
+
+def test_cleanup_backup() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("content\n")
+    backup_file = test_file.with_suffix(".py.backup")
+    backup_file.write_text("backup\n")
+    
+    cleanup_backup(str(test_file))
+    assert not backup_file.exists()