feat(tauri): add Windows taskbar overlay badge for git sync status

This change introduces a Windows-only taskbar badge that displays the
ahead+behind count as a rendered overlay icon. The UI now updates
the badge whenever the repository sync status changes, and the git
diff commands are adjusted to avoid external diff/text conversion noise.

- Add badge rendering and Tauri command for setting overlay icon
- Wire badge updates into the Svelte app based on git status
- Update git diff invocations to disable ext-diff/textconv

src-tauri/src/git.rs

@@ -1302,6 +1302,8 @@ pub fn compare_commits(
         &repo,
         [
             "diff",
+            "--no-ext-diff",
+            "--no-textconv",
             "-M",
             FULL_FILE_DIFF_CONTEXT,
             from_hash.as_str(),
@@ -1344,7 +1346,14 @@ pub fn diff_file_against_working_tree(
     )?;
     let patch_output = run_git_with_paths(
         &repo,
-        &["diff", "-M", FULL_FILE_DIFF_CONTEXT, commit_hash.as_str()],
+        &[
+            "diff",
+            "--no-ext-diff",
+            "--no-textconv",
+            "-M",
+            FULL_FILE_DIFF_CONTEXT,
+            commit_hash.as_str(),
+        ],
         std::slice::from_ref(&file),
     )?;
 
@@ -1400,6 +1409,8 @@ pub fn compare_file_to_head(
         &repo,
         &[
             "diff",
+            "--no-ext-diff",
+            "--no-textconv",
             "-M",
             FULL_FILE_DIFF_CONTEXT,
             commit_hash.as_str(),
@@ -1472,6 +1483,8 @@ pub fn compare_file_to_parent(
         &repo,
         &[
             "diff",
+            "--no-ext-diff",
+            "--no-textconv",
             "-M",
             FULL_FILE_DIFF_CONTEXT,
             from_hash.as_str(),