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/.gitignore

@@ -24,3 +24,4 @@ next-env.d.ts
 npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
+*.tsbuildinfo