feat: add expiration to posts in contract

[bochaco]

Add post expiration to the bulletin board

Posts now expire after 3 minutes:

• Once a post has expired, anyone can remove it — not just the original poster.
• Before expiration, only the owner can take it down.

Changes made:

• Contract:

  • Added a postTimestamp ledger field to track when each post expires.
  • post now takes a nowSecs timestamp argument and validates it is more recent than the previous post (prevents replay).
  • takeDown now allows removal if the caller is the owner or the block time has passed the expiration timestamp — whichever comes first.

• API

  • post() automatically passes the current time (epoch seconds) to the contract, so callers don't need to handle it.

• Tests

  • Added setBlockTime to the simulator to allow tests to control the on-chain clock.
  • New test cases:
    • A non-owner cannot remove a post before it expires.
    • The original poster can still take down their post early.
    • Anyone can remove a post after it expires.
    • The sequence counter increments correctly through both the normal and expiration-based take-down paths.

• CLI

  • The "display derived state" option now shows two additional lines when a board is occupied:
    • Post age — how long ago the message was posted.
    • Expires in / Expired X ago — how much time is left, or how long ago it expired (with a note that the post can be removed by anyone once expired).

• UI

  • The post card's action bar now shows the expiration date/time in the user's local timezone (e.g. "Expires 4/21/26, 4:30 PM") on the right side when a board is occupied.
  • When a non-owned post expires, it automatically enables the button to take down the post.