refactor(frontend): introduce promoted posts & safer "burn" mechanism

[luciorubeens]

Based on Jae's feedback , introduced the concept of "promoted posts" to replace the PHOTON input field.

• Replaced the manual burn/send amount input with a promote toggle.
• Users can no longer accidentally send arbitrary amounts, instead they choose between a standard or a promotion amount.
• The promote amount is configurable via environment variables
• Added a "promoted" badge for posts that meet the promotion threshold.

Fixes #496