feat(git): add diff, branch, checkout, push, pull to Git sidebar panel

Extends GitOperations.js with diff/branches/checkoutBranch/push/pull,
wires the 5 new IPC handlers in main.js (reusing the existing dir
resolution), whitelists the new channels in preload.js, and fixes the
Git sidebar panel's previously dead _gitDiff callback by wiring up a
diff view, branch list/create/checkout UI, and push/pull buttons.
Resolves Task 5, which deferred this work to this task.

Amit Haridas

src/main.js

@@ -5294,6 +5294,26 @@ ipcMain.handle('git-log', async () => {
   const dir = currentFile ? path.dirname(currentFile) : process.cwd();
   return GitOperations.log(dir);
 });
+ipcMain.handle('git-diff', async (event, { file } = {}) => {
+  const dir = currentFile ? path.dirname(currentFile) : process.cwd();
+  return GitOperations.diff(dir, file);
+});
+ipcMain.handle('git-branches', async () => {
+  const dir = currentFile ? path.dirname(currentFile) : process.cwd();
+  return GitOperations.branches(dir);
+});
+ipcMain.handle('git-checkout', async (event, { name, isNew } = {}) => {
+  const dir = currentFile ? path.dirname(currentFile) : process.cwd();
+  return GitOperations.checkoutBranch(dir, name, isNew);
+});
+ipcMain.handle('git-push', async () => {
+  const dir = currentFile ? path.dirname(currentFile) : process.cwd();
+  return GitOperations.push(dir);
+});
+ipcMain.handle('git-pull', async () => {
+  const dir = currentFile ? path.dirname(currentFile) : process.cwd();
+  return GitOperations.pull(dir);
+});
 
 // ============================================
 // Snippets IPC Handlers