refactor(gui_2,patch_modal): merge diff_viewer ops into gui_2; data classes (DiffHunk/DiffFile) move to patch_modal.py alongside PendingPatch; git rm src/diff_viewer.py

Per spec FR1 + Phase 1.4 + architecture feedback (data != view):
  - Data classes DiffHunk, DiffFile -> src/patch_modal.py (alongside PendingPatch; all patch-domain data)
  - Operations parse_diff/parse_hunk_header/get_line_color/apply_patch_to_file (called by gui_2) -> src/gui_2.py
  - GUI is a pure view; data lives elsewhere; no new files per AGENTS.md

Tests: tests/test_diff_viewer.py imports from src.gui_2 (parse_diff/apply_patch_to_file) and src.patch_modal (DiffFile/DiffHunk).

src/patch_modal.py

@@ -2,6 +2,21 @@ from dataclasses import dataclass, field
 from typing      import Optional, Callable, List
 
 
+@dataclass
+class DiffHunk:
+ header:    str
+ lines:     List[str]
+ old_start: int
+ old_count: int
+ new_start: int
+ new_count: int
+
+@dataclass
+class DiffFile:
+ old_path: str
+ new_path: str
+ hunks:    List[DiffHunk] = field(default_factory=list)
+
 @dataclass
 class PendingPatch:
  patch_text:   str = ""