task: onboarding rewards slide still promises a 'priciest plot' board that #224 removed

## What needs to be done?
The onboarding rewards slide still tells new players to compete for the "priciest plot", a board that no longer exists. Since #224 (merged 9 Sep) the ranks page has LAND, EMPIRE and CAMPAIGN; the TYCOONS tab — the one "priciest plot" pointed at — was replaced. The FAQ was updated in the same PR (`faq/content.ts:72` names the three live boards through `BOARD_LABELS`), the carousel was not.

**Verified by reading the source** at `main` (`967bc81`):

- `apps/web/src/components/Overlays/IntroScreen.tsx:159` — `'Biggest empire, most pixels, priciest plot. Pick your flex. When a campaign runs, top players get paid in USDT.'`
- `apps/web/src/lib/maps/leaderboards.ts:28` — `LeaderboardTab = "AREA" | "EMPIRE" | "CAMPAIGN"`; the priciest-pixel ranking function is still in the file but nothing renders it.
- `apps/web/src/lib/share.ts:45` — already treats `'TYCOONS'` as a legacy value older shares may carry.

**Impact:** the slide is shown once, full-screen, to every new player, and it names a way to win that cannot pay. Same family as the "Claim daily rewards" line #183 removed from the same slide — a promise the boards do not honour.

## Suggested fix
One string. The third flex should be the CAMPAIGN board's — growth inside the window — e.g. "Biggest empire, most land, biggest climb during a campaign. Pick your flex. When a campaign runs, top players get paid in USDT." Wording is the PM's call, especially since `CAMPAIGN`'s own label is still a placeholder (`leaderboards.ts:39-42`). The existing copy-guard test on this slide (`__tests__/components/Overlays/IntroScreen.test.tsx`) keeps "when a campaign runs" and forbids daily/claim/withdraw; add one assertion that the deck never says "priciest".

## Definition of done
- [ ] Rewards slide names only boards that exist (LAND, EMPIRE, CAMPAIGN in whatever words)
- [ ] Copy-guard test rejects "priciest"
- [ ] "When a campaign runs … get paid" stays intact

## Size
S — under an hour once the wording is chosen.

## OPEN
- The words for the CAMPAIGN flex — same open question as the tab label itself.