web: say the pricing promise plainly on the landing page

The pricing head asked the reader to start free and go Pro when the grain
matters, which is our joke rather than their question. It now says what
the plans actually are: free to use, forever, and Pro only when they're
ready for more.

The Vietnamese line moves with it — "Miễn phí dùng mãi. Chỉ lên Pro khi
bạn cần nhiều hơn." — so the two languages still promise the same thing.

ponytail: one string on each side of the existing c({ en, vi }) pair, no
new copy key and no layout change.

Verified:
- npx tsc --noEmit clean.
- Against the rebuilt production bundle on :8090, a probe reads #pricing
  h2 in both languages: EN "Free to use, forever. Upgrade to Pro only
  when you're ready for more.", VI "Miễn phí dùng mãi. Chỉ lên Pro khi
  bạn cần nhiều hơn.", VI overflow 0px — 3/3.
- landing-test, lp-probe, subnav-anchor-probe, pro-gate-test all rc=0
  fails=0; screenshot of the section in both languages shows the head
  wrapping over two lines above the plan cards, nothing clipped.

docker/frontend/src/Landing.tsx

@@ -1022,7 +1022,7 @@ export function Landing() {
         <div className="lp-wrap">
           <div className="lp-sec-head">
             <span className="lp-eyebrow">{c({ en: 'Pricing', vi: 'Bảng giá' })}</span>
-            <h2>{c({ en: 'Start free. Go Pro when the grain matters.', vi: 'Bắt đầu miễn phí. Lên Pro khi bạn cần hạt phim.' })}</h2>
+            <h2>{c({ en: 'Free to use, forever. Upgrade to Pro only when you\'re ready for more.', vi: 'Miễn phí dùng mãi. Chỉ lên Pro khi bạn cần nhiều hơn.' })}</h2>
           </div>
           <div className="lp-price">
             <article className="lp-plan">