feat: show diff view for AI CV rewrites

The master-CV rewrite preview replaced text without showing what
changed -- the teardown flagged this as the biggest unmanaged AI risk
(a rewrite silently upgrading "assisted with migration" to "led
migration" was invisible). Add a "Show changes" toggle on the rewrite
preview panel that renders a word-level diff (before = current master
text or the targeted section's stored content, after = the AI's
rewrite) instead of the flat replacement text.

Defaults to off: an existing test proved diff-by-default breaks the
familiar plain-text read (word-fragmented spans aren't matchable as
one block), and it's a better UX default regardless -- read normally,
opt into the diff when you want the trust signal.

Uses the `diff` package (word-level diffWords) rather than hand-rolled
LCS; no existing dependency covers this, and it's a solved problem.

job-tracker-ui/src/text-diff.test.tsx

@@ -0,0 +1,26 @@
+import React from "react";
+import { render, screen } from "@testing-library/react";
+import "@testing-library/jest-dom";
+import TextDiff from "./components/TextDiff";
+
+describe("TextDiff", () => {
+  it("renders unchanged text without strike-through or highlight styling", () => {
+    render(<TextDiff before="Backend engineer." after="Backend engineer." />);
+    expect(screen.getByText("Backend engineer.")).toBeInTheDocument();
+  });
+
+  it("marks removed words with a distinct style from unchanged text", () => {
+    render(<TextDiff before="Assisted with the migration." after="Led the migration." />);
+    const removed = screen.getByText("Assisted with", { selector: "span" });
+    const unchanged = screen.getByText(/the migration\./, { selector: "span" });
+    // jsdom doesn't resolve emotion's generated CSS cascade for getComputedStyle, so assert the
+    // component branched into a different (MUI-generated) class for removed vs. unchanged text
+    // rather than the literal computed decoration value.
+    expect(removed.className).not.toBe(unchanged.className);
+  });
+
+  it("treats an empty before as an entirely new addition", () => {
+    render(<TextDiff before="" after="Brand new summary." />);
+    expect(screen.getByText(/Brand new summary\./)).toBeInTheDocument();
+  });
+});