Sign-up's username and taken-email sentences live in one module, read by every way an account is made #5592

For: Making an account with a bad username or a taken email reads the same wherever it is said, because it is said from one place. Today each sentence below is typed out separately in src/actions/first_admin.ts, src/web/pages/claim.ts, src/web/pages/everyday.ts, src/web/pages/signup-passkey.ts, src/web/pages/site.ts, so the next edit to one copy makes them drift.

Done:

• [ ] src/actions/signup_words.ts exports these sentences (new, or the existing module of that concept if one already owns similar words; say which in the pull):

- "Usernames are letters, digits, dots and dashes." - "That email already has an account."

• [ ] Every file named imports them, and grep -rn for each sentence under src/ finds only the module.
• [ ] Answers and pages are byte-for-byte unchanged: every existing test passes as it is.
• [ ] A new test/signup-words.test.ts asserts each exported sentence is non-empty and that each file named no longer contains the literal (read the source as text). It fails if a copy is typed back.
• [ ] Before pushing, run test/signup-words.test.ts, the tests of the files you touched, bun x tsc --noEmit and, if src/web changed, bun scripts/design-lint.ts. A red test elsewhere that main also has is named in the pull, not a reason to stop.

Files: src/actions/signup_words.ts, src/actions/first_admin.ts, src/web/pages/claim.ts, src/web/pages/everyday.ts, src/web/pages/signup-passkey.ts, src/web/pages/site.ts, test/signup-words.test.ts. Risk: a pure move; change no wording. Touch only the lines the sentences are on.