Settings › Model speaks the home page's language #9800

Bring /settings/model (modelPage in src/web/pages/model.ts) into line with the home page, the same way Settings › Backups did in #9754.

Done looks like:

• [x] The page's card( around the subscription form becomes a <section class="settings-block"> with an <h2>. A grant an admin gave is one row(). With nothing, one plain sentence says what the account may spend and how to add a subscription.
• [x] A token is still typed once and never shown again. The existing tests pass unchanged, including the timing budget: test/model-*.test.ts and test/model-grant-pages.test.ts.
• [x] A new test renders the page for a new account and for one with a grant, checks for the heading, and finds no .card. It fails on main.
• [x] docs/design/areas/settings.md records the shape. bun scripts/design-lint.ts is clean and bun test passes.

Files: src/web/pages/model.ts, docs/design/areas/settings.md, test/. Risks: ceilingCard and grantCard are also drawn on an account's page by accounts.ts. Leave those two as they are here, and change only what modelPage draws.

Reviewed pull #9857 at its head: ticked The page's card( around the subscription form becomes a <section class="settings-block"> with an <h2>. A grant an admin gave is one row(). With nothing, one plain sentence says what the account may spend and how to add a subscription.; A token is still typed once and never shown again. The existing tests pass unchanged, including the timing budget: test/model-*.test.ts and test/model-grant-pages.test.ts.; A new test renders the page for a new account and for one with a grant, checks for the heading, and finds no .card. It fails on main.; docs/design/areas/settings.md records the shape. bun scripts/design-lint.ts is clean and bun test passes..