feat: email signature

[thaninbew]

Description

Wired up the frontend Email Editor to the backend template endpoints and fixed the signature/CTA state loading.

Changes Made

• Frontend changes
• Added template fetching on mount for all 3 email types.
• Fixed the duplicating signature bug on reload by wrapping metadata in hidden HTML comments to safely parse out the body text.
• Added all missing signature inputs (Name, Email, Pronouns, Socials, etc.) and updated defaults to the official FCC links.

Testing & Verification

• Manual testing completed
• Lint passes

Future Improvements/Notes

• Left a placeholder text input for the signature image URL for now. I'm waiting on the S3/CDN feature from Hannah's ticket before swapping this to a real input.

Related Issues

Closes #114