Offer page & result copy editor (config-driven status messages)

[hamin2006]

Context

The applicant-facing offer letter and status copy are effectively uneditable by admins: the offer letter lives in raw form config JSON (edited via SQL every season), and most other decision screens are hardcoded in components. Settings shows a preview but has no editor.

Current behavior

• Offer letter: config.application.pages.offerPage.content rendered by offerPage.tsx with {{...}} templating over app details + lpContext (teams etc.). No editor anywhere; settings preview only renders it.
• Status labels/descriptions: config.application.status[] — labels feed pickers; description has no consumer; no editor.
• Hardcoded screens (not even SQL-editable): rejected/closed/submitted copy in MainResultPage.tsx/GenericResult, accepted text in acceptedResult.tsx, submitted celebration in submittedResult.tsx, pending in pendingResult.tsx, decline text in offerPage.tsx.
• Unknown statuses render the raw status id string to the applicant.

Desired behavior

• Editor (natural home: Settings) for the offer-page content and per-status applicant-facing message, with live preview using the existing per-app preview picker.
• Move hardcoded decision copy into per-status config with component defaults as fallback (so unconfigured forms behave exactly as today).
• Support the existing {{tag}} substitution in edited copy; preview reflects substitutions with a chosen applicant.

Acceptance criteria

• Offer letter editable in UI with template-tag help and live preview
• Accepted/rejected/submitted/pending/declined messages editable per status where applicable, defaulting to today's text when unset
• Unknown/terminal statuses never render a raw id string to applicants
• Zero-migration for existing forms (defaults fill in)