Add Project Launch Date

[Naomi-Gift]

The contract stores creation time but not actual product launch date. Frontends may need to show when a project launched.

Acceptance criteria:

• Project registration supports optional launch timestamp.
• Project owner can update launch timestamp.
• Invalid future/past bounds are handled according to documented rules.
• Tests cover registration and update behavior.

[mubking]

please i would like to work on this issue