fix(i18n): hold every refusal reason by type, and refuse what the scan cannot read

Review round 1 of the refusal contract: it claimed more than it held.

- The other reason carriers get their Literal too (BlockedUrl, ClipError,
  FolderError, the playlist and sign-in refusal helpers), so the type
  checker refuses a reason passed by variable or an unknown literal, and
  the contract reads one shape.
- A literal reason in a detail dict built anywhere counts as a reason of
  its code; a code the scan cannot read, a refused() whose reason it
  cannot read, and a new reason passed by variable each fail the contract
  until listed with what holds them.
- A sign-in refusal key the backend no longer sends is caught too.
- localizeDetail's numbers (download cap in GB, upload cap in bytes) and
  the shared-sentence fallbacks are tested in both languages.
- One Literal parser in the contract; a stale comment fixed.

backend/app/refusal.py

@@ -11,7 +11,8 @@ A test (`test_http_details.py`) reads every `HTTPException` in the app and fails
 and the frontend's contract test (`i18n/backendIds.contract.test.ts`) requires every `code` a detail
 carries to be in one record of `lib/api/refusalCodes.ts`, and every reason it can be sent with — a
 literal, or a `Literal` type when it travels in a variable — to have its sentence in both languages.
-So the rule is enforced, not described.
+A code or reason that scan cannot read fails it until it is listed with what holds it. So the rule is
+enforced, not described.
 """
 
 from fastapi import HTTPException