The new repository form at /new fits a 390 px phone: no sideways scroll #9830

From #9813's open box. /new now speaks the home page's language (#9813); nothing yet pins that it fits a phone. Follow test/apps-fit.test.ts (#9371) and test/bets-card-fit.test.ts (#9286).

Done looks like:

• [x] A new test/new-repo-fit.test.ts renders /new signed in, with a 300-character unbroken owner or template name seeded where the form shows one, and asserts that no inline width/min-width and no rule the page ships fixes a width over 390 px, and that long-text containers carry overflow-wrap: anywhere (or break-word). Where a Chromium starts, as in apps-fit, it asserts the page does not scroll sideways at 390 px.
• [x] Anything the test finds is fixed in src/web/pages/ for /new (the file serving newRepoPage); any new class is named in docs/design/areas/forms.md, not docs/DESIGN.md. If the page already fits, the test pins it and the pull says so.
• [x] test/new-repo-form.test.ts and test/new-repo-timing.test.ts pass; bun scripts/design-lint.ts is clean.

Files: test/new-repo-fit.test.ts, maybe the /new page and docs/design/areas/forms.md. Risks: keep the form's fields, names and same-origin POST to /new unchanged.

Reviewed pull #9833 at its head: ticked A new test/new-repo-fit.test.ts renders /new signed in, with a 300-character unbroken owner or template name seeded where the form shows one, and asserts that no inline width/min-width and no rule the page ships fixes a width over 390 px, and that long-text containers carry overflow-wrap: anywhere (or break-word). Where a Chromium starts, as in apps-fit, it asserts the page does not scroll sideways at 390 px.; Anything the test finds is fixed in src/web/pages/ for /new (the file serving newRepoPage); any new class is named in docs/design/areas/forms.md, not docs/DESIGN.md. If the page already fits, the test pins it and the pull says so.; test/new-repo-form.test.ts and test/new-repo-timing.test.ts pass; bun scripts/design-lint.ts is clean..