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/GitDropdown.tsx

@@ -173,7 +173,6 @@ function SyncDropdownWithSyncDir({ syncDir }: { syncDir: string }) {
     { type: 'separator' },
     {
       label: 'Push',
-      hidden: !hasRemotes,
       leftSlot: <Icon icon="arrow_up_from_line" />,
       waitForOnSelect: true,
       async onSelect() {
@@ -192,7 +191,6 @@ function SyncDropdownWithSyncDir({ syncDir }: { syncDir: string }) {
     },
     {
       label: 'Pull',
-      hidden: !hasRemotes,
       leftSlot: <Icon icon="arrow_down_to_line" />,
       waitForOnSelect: true,
       async onSelect() {