Add split view and diff view for comparing files side by side

Adds two new features activated via tab context menu:
- Split View: view two open files side by side with independent scrolling
- Diff View: compare two files with color-coded differences, aligned spacers,
  synchronized scrolling, and hunk navigation (F7/Shift+F7)

All split/diff code paths are gated behind viewMode checks with zero impact
on normal single-pane performance. Includes new IPC channels (get-lines-for-file,
diff-compute), SecondaryViewer with its own cache/pool, and keyboard shortcuts
(Ctrl+Shift+D toggle diff, Escape close).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ozgesolidkey

package-lock.json

@@ -30,13 +30,15 @@
   },
   "devDependencies": {
     "@electron/rebuild": "^3.7.2",
+    "@types/diff": "^7.0.2",
     "@types/node": "^20.10.0",
     "electron": "^28.0.0",
     "electron-builder": "^24.9.1",
     "typescript": "^5.3.0",
     "vitest": "^4.0.18"
   },
   "dependencies": {
+    "diff": "^8.0.3",
     "marked": "^17.0.1",
     "node-pty": "^1.1.0",
     "xterm": "^5.3.0",