Fix git pull conflicts with pull.ff=only and improve commit UX

- Replace git pull with fetch + merge to avoid conflicts with global
  git config (e.g. pull.ff=only) and background fetch --all
- Disable commit/commit+push buttons when message is empty
- Always show Push/Pull menu items even without remotes configured
- Default remote name to 'origin' when adding a new remote

src-web/components/git/showAddRemoteDialog.tsx

@@ -3,12 +3,12 @@ import { gitMutations } from '@yaakapp-internal/git';
 import { showPromptForm } from '../../lib/prompt-form';
 import { gitCallbacks } from './callbacks';
 
-export async function addGitRemote(dir: string): Promise<GitRemote> {
+export async function addGitRemote(dir: string, defaultName?: string): Promise<GitRemote> {
   const r = await showPromptForm({
     id: 'add-remote',
     title: 'Add Remote',
     inputs: [
-      { type: 'text', label: 'Name', name: 'name' },
+      { type: 'text', label: 'Name', name: 'name', defaultValue: defaultName },
       { type: 'text', label: 'URL', name: 'url' },
     ],
   });