feat(git): sync with a remote — fetch, pull and push

Add remote sync to the Git panel: set/edit an origin URL, then fetch, pull
(fast-forward) and push, with live ahead/behind counts against the
remote-tracking branch.

Backend:
- git_ops: remote_status (git2 ahead/behind + upstream), set_remote, and
  fetch/pull/push that shell out to the system git CLI so they reuse the
  user's existing credentials (SSH agent, keychain / credential helpers)
  instead of reimplementing libgit2 credential callbacks. Pull is --ff-only
  to avoid leaving a conflicted tree from the GUI; push uses --set-upstream.
- New commands git_remote_status/set_remote/fetch/pull/push, registered.
- Test: push to a bare local remote, then assert upstream + ahead/behind.

Frontend:
- RemoteStatus type, api wrappers, store state (remote, syncing) with
  refreshRemote wired into refreshGit; pull reloads diagrams + open buffer.
- GitPanel remote section: URL, ahead/behind badges, Fetch/Pull/Push, and a
  set-remote dialog.

cargo test (7 passing incl. roundtrip), svelte-check and build all green.

src-tauri/src/commands.rs

@@ -6,7 +6,7 @@ use tauri::State;
 use crate::db::{self, ProjectRecord};
 use crate::diagram::{self, DiagramInfo};
 use crate::error::{AppError, Result};
-use crate::git_ops::{self, BranchInfo, CommitInfo, FileStatus};
+use crate::git_ops::{self, BranchInfo, CommitInfo, FileStatus, RemoteStatus};
 use crate::project::{self, ProjectConfig};
 use crate::state::AppState;
 
@@ -176,6 +176,37 @@ pub fn git_checkout_branch(path: String, name: String) -> Result<()> {
     git_ops::checkout_branch(Path::new(&path), &name)
 }
 
+// ---- Remote sync ----------------------------------------------------------
+
+#[tauri::command]
+pub fn git_remote_status(path: String) -> Result<RemoteStatus> {
+    git_ops::remote_status(Path::new(&path))
+}
+
+#[tauri::command]
+pub fn git_set_remote(path: String, url: String) -> Result<()> {
+    let url = url.trim();
+    if url.is_empty() {
+        return Err(AppError::other("remote URL cannot be empty"));
+    }
+    git_ops::set_remote(Path::new(&path), url)
+}
+
+#[tauri::command]
+pub fn git_fetch(path: String) -> Result<String> {
+    git_ops::fetch(Path::new(&path))
+}
+
+#[tauri::command]
+pub fn git_pull(path: String) -> Result<String> {
+    git_ops::pull(Path::new(&path))
+}
+
+#[tauri::command]
+pub fn git_push(path: String) -> Result<String> {
+    git_ops::push(Path::new(&path))
+}
+
 // ---------------------------------------------------------------------------
 // Export helpers & settings
 // ---------------------------------------------------------------------------