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/renderer.js

@@ -1751,6 +1751,14 @@ document.addEventListener('DOMContentLoaded', async () => {
             message,
           }),
         gitLog: () => ipcRenderer.invoke('git-log'),
+        gitBranches: () => ipcRenderer.invoke('git-branches'),
+        gitCheckout: (name, isNew) =>
+          ipcRenderer.invoke('git-checkout', {
+            name,
+            isNew,
+          }),
+        gitPush: () => ipcRenderer.invoke('git-push'),
+        gitPull: () => ipcRenderer.invoke('git-pull'),
       }),
   });
   sidebarManager.registerPanel('snippets', {