Add parent name to teacher email sign-off

[sjiang1]

Goal

In the teacher email draft, include a parent name alongside the student name in the sign-off, so the teacher sees that the parent sent it on the kid's behalf.

Builds on #1 / PR #2 (the deterministic `composeTeacherEmail` formatter).

Where the name comes from

Add a parent name to the student data model (src/app/students.ts):

• Add an optional parentName?: string to the Student type.
• Seed/Redis: the public single-student seed (data/students.json, Mozart X) can carry a fictional parent (e.g. Mama Bear); real value lives only in prod Redis, per the de-identify-via-runtime-config approach.
• Thread it through: teacher/page.tsx passes primary.parentName into composeTeacherEmail (extend ComposeInput).

Sign-off format

TBD — decide during implementation. Candidates:

• — {parentName} & {studentName}
• Thanks! / — {studentName} / (sent by {parentName})
• — {parentName} (for {studentName})

Fallback

If no parentName is set (e.g. single-student deploy with none configured), the sign-off stays exactly as today: Thanks! / — {studentName}. No regression.

Out of scope

• A separate parent/guardian entity or account (the linked-account concept is unrelated here).
• Email delivery — still copy-to-clipboard only.