Update Fall 2026 recruitment timeline

[Hilo-Hilo]

Summary

• replace the Fall 2026 recruitment graphic with the updated schedule
• add the full event timeline and revised deadline copy to the signup page
• refresh the recruitment banner and Instagram update links
• prevent the poster from being cropped on desktop and mobile

Verification

• npm test
• changed-file ESLint
• npm run build
• git diff --check
• desktop and 390 px mobile visual QA
• independent pre-commit review passed

Note: full-project lint still reports the repository's existing two react-refresh/only-export-components errors in unrelated UI files; changed files lint clean.