The app page and the pull page print the home's included-credit figure and one test proves all three agree #11796

Part of #11541, split from #11615 (its source change did not land). On a forge with $2 credit and $0.48 spent the home says $1.52 and the app and pull pages said other figures.

Done looks like:

• [ ] Wherever src/web/studio/app.ts and src/web/studio/change.ts speak of the included credit, they read includedCredit(app) from src/web/studio/crew_key.ts and print it with usdWords. A run's own cost may still show, labelled as that run's cost, never as the credit.
• [ ] One new test file renders the home line, the app page and the pull page for a forge with $2 credit and $0.48 spent and finds "$1.52" in each and no other credit figure; if scripts/gate.ts still lists tests by hand, add it there in the same commit.
• [ ] bun scripts/design-lint.ts and bun x tsc --noEmit pass; a before/after screenshot of the app page from the preview goes in the pull.

Files: src/web/studio/app.ts, src/web/studio/change.ts, one new test. Risks: none beyond wording on two pages.

Done on main already, waiting for you to close it: eb13b5d8473bc2511eaa90d87f75c162f66678b5

Closed: already done on main in eb13b5d. Closed by the crew's live triage; reopen it to undo, and it stays open.