fix(KB-109): fix FileEditor layout and add component tests

- Fix FileEditor to properly use full width with box-sizing and min-width constraints
- Add visible focus and selection states for the textarea-based editor
- Ensure FileEditor correctly responds to theme changes (dark/light mode)
- Add comprehensive FileEditor component tests (60 test lines)
- Include changeset for patch release of @dustinbyrne/kb

packages/dashboard/app/components/FileEditor.test.tsx

@@ -0,0 +1,60 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen, fireEvent } from "@testing-library/react";
+import { FileEditor } from "./FileEditor";
+
+describe("FileEditor", () => {
+  it("renders textarea with correct class names", () => {
+    render(<FileEditor content="" onChange={vi.fn()} />);
+    const textarea = screen.getByRole("textbox");
+    expect(textarea.classList.contains("file-editor-container")).toBe(true);
+    expect(textarea.classList.contains("file-editor-textarea")).toBe(true);
+  });
+
+  it("renders with content prop value", () => {
+    const content = "const x = 42;";
+    render(<FileEditor content={content} onChange={vi.fn()} />);
+    const textarea = screen.getByRole("textbox") as HTMLTextAreaElement;
+    expect(textarea.value).toBe(content);
+  });
+
+  it("calls onChange when text is modified", () => {
+    const onChange = vi.fn();
+    render(<FileEditor content="" onChange={onChange} />);
+    const textarea = screen.getByRole("textbox");
+    
+    fireEvent.change(textarea, { target: { value: "new content" } });
+    
+    expect(onChange).toHaveBeenCalledWith("new content");
+  });
+
+  it("respects readOnly prop", () => {
+    render(<FileEditor content="readonly content" onChange={vi.fn()} readOnly />);
+    const textarea = screen.getByRole("textbox") as HTMLTextAreaElement;
+    expect(textarea.readOnly).toBe(true);
+  });
+
+  it("has correct aria-label based on filePath prop", () => {
+    const filePath = "src/components/App.tsx";
+    render(<FileEditor content="" onChange={vi.fn()} filePath={filePath} />);
+    const textarea = screen.getByRole("textbox");
+    expect(textarea.getAttribute("aria-label")).toBe(`Editor for ${filePath}`);
+  });
+
+  it("has default aria-label when filePath is not provided", () => {
+    render(<FileEditor content="" onChange={vi.fn()} />);
+    const textarea = screen.getByRole("textbox");
+    expect(textarea.getAttribute("aria-label")).toBe("File editor");
+  });
+
+  it("has spellCheck disabled", () => {
+    render(<FileEditor content="" onChange={vi.fn()} />);
+    const textarea = screen.getByRole("textbox");
+    expect(textarea.getAttribute("spellcheck")).toBe("false");
+  });
+
+  it("is not readOnly by default", () => {
+    render(<FileEditor content="" onChange={vi.fn()} />);
+    const textarea = screen.getByRole("textbox") as HTMLTextAreaElement;
+    expect(textarea.readOnly).toBe(false);
+  });
+});