fix: use server-computed endsAt for exploration timer to prevent clock drift

Exploration timers were showing more time than the area's stated duration
when the server clock was ahead of the client clock. The timer now uses
the server-provided endsAt timestamp directly instead of deriving startedAt
from it, making countdowns immune to client/server clock skew. Old saves
without endsAt fall back to the previous startedAt-based calculation.

Closes #53

packages/types/src/interfaces/exploration.ts

@@ -72,6 +72,12 @@ interface ExplorationAreaState {
    */
   startedAt?: number;
 
+  /**
+   * Unix timestamp when the exploration will complete (server-computed, used for
+   * accurate client-side countdown that is immune to client/server clock drift).
+   */
+  endsAt?: number;
+
   /**
    * True after the first successful collect — used for codex unlock detection.
    */