fix(app): preserve file line endings and UTF-8 BOM (#2277)

* fix(app): preserve file line endings and UTF-8 BOM

* refactor(app): simplify line ending preservation

Let the editor parse newline variants and serialize with the file's first separator. Mixed-ending files become uniform on edit without a separate normalization layer.

---------

Co-authored-by: Mohamed Boudra <boudra.moha@gmail.com>

packages/app/e2e/file-editing.spec.ts

@@ -239,6 +239,29 @@ test.describe("CodeMirror workspace file editing", () => {
     await expect(editor(page)).toContainText("const afterReconnect = 9;");
   });
 
+  test("preserves a UTF-8 BOM and uses the first line separator after saving", async ({
+    page,
+    withWorkspace,
+  }) => {
+    const workspace = await withWorkspace({ prefix: "file-editing-encoding-" });
+    const sourcePath = path.join(workspace.repoPath, "windows.ts");
+    await writeFile(
+      sourcePath,
+      Buffer.from("\uFEFFconst initial = true;\r\nconst mixed = true;\n", "utf8"),
+    );
+    await workspace.navigateTo();
+    await openWorkspaceFile(page, "windows.ts");
+
+    await replaceEditorText(page, "const saved = true;\nconst normalized = true;\n");
+    await editor(page).press("Control+s");
+
+    const expected = Buffer.from(
+      "\uFEFFconst saved = true;\r\nconst normalized = true;\r\n",
+      "utf8",
+    ).toString("hex");
+    await expect.poll(async () => (await readFile(sourcePath)).toString("hex")).toBe(expected);
+  });
+
   test("warns before closing a panel with an unsaved draft", async ({ page, withWorkspace }) => {
     const workspace = await withWorkspace({ prefix: "file-editing-draft-" });
     const sourcePath = path.join(workspace.repoPath, "draft.ts");