Threads posts over 500 chars were saved + scheduled successfully and only
failed inside the publish job. The frontend already showed the 537|500 badge
but `canSchedule` ignored content length, so Schedule and Post Now stayed
enabled. Backend `UpdatePostRequest` only capped at 63206 (Facebook's max),
not per-platform.
- Add `Platform::contentOverflow()` as the single source of truth and reuse it
from `HasSocialHttpClient::validateContentLength` (publish-time).
- New `ContentFitsPlatformLimits` rule applied to the `content` field on
`App\\UpdatePostRequest`, `Api\\UpdatePostRequest`, and `Api\\StorePostRequest`
via `Rule::when(...)` so drafts are not blocked.
- Rule dedupes per platform (two Threads accounts -> one error) and reports
the platform label, hard cap, and overage via i18n.
- Edit.vue feeds `contentLengthOverflows` into `canSchedule` and lists each
offending platform in `postActionTooltip` using the existing
`getPlatformLabel` resolver.
