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/package.json

@@ -22,6 +22,7 @@
     "@types/react-dom": "^19.2.3",
     "axios": "^1.15.0",
     "date-fns": "^4.1.0",
+    "diff": "^9.0.0",
     "next": "^16.2.10",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",