fix: use server-computed endsAt for exploration timer to prevent clock drift (#68)

## Summary

- Exploration timers showed more time than the area's stated duration when the server clock was ahead of the client's
- The timer was derived from `startedAt = endsAt - durationMs`, then computed as `durationSeconds - (clientNow - startedAt) / 1000` — any server/client clock skew directly inflated the result
- Now stores `endsAt` (the server-computed completion timestamp) directly in `ExplorationAreaState` and computes the timer as `(endsAt - Date.now()) / 1000`, which is immune to clock drift
- Old saves without `endsAt` fall back gracefully to the previous `startedAt`-based calculation

## Test plan

- [ ] Start a new exploration — timer should show exactly the area's stated duration (no more "1h area shows 1h15m")
- [ ] Refresh the page mid-exploration — timer should resume from the correct remaining time (using server-anchored `endsAt`)
- [ ] Old saves with `startedAt` but no `endsAt` should still display a timer via the fallback path

Closes #53

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #68
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/web/src/context/gameContext.tsx

@@ -52,7 +52,6 @@ import {
   transcend as transcendApi,
 } from "../api/client.js";
 import { CODEX_ENTRIES } from "../data/codex.js";
-import { EXPLORATION_AREAS } from "../data/explorations.js";
 import { RECIPES } from "../data/recipes.js";
 import {
   RESOURCE_CAP,
@@ -1671,14 +1670,6 @@ export const GameProvider = ({
   const startExploration = useCallback(async(areaId: string) => {
     try {
       const response = await startExplorationApi({ areaId });
-      const areaData = EXPLORATION_AREAS.find((a) => {
-        return a.id === areaId;
-      });
-      if (areaData === undefined) {
-        return;
-      }
-      // eslint-disable-next-line stylistic/no-mixed-operators -- duration * 1000 is clear
-      const startedAt = response.endsAt - areaData.durationSeconds * 1000;
       setState((previous) => {
         if (previous?.exploration === undefined) {
           return previous;
@@ -1689,7 +1680,11 @@ export const GameProvider = ({
             ...previous.exploration,
             areas: previous.exploration.areas.map((a) => {
               return a.id === areaId
-                ? { ...a, startedAt: startedAt, status: "in_progress" as const }
+                ? {
+                  ...a,
+                  endsAt: response.endsAt,
+                  status: "in_progress" as const,
+                }
                 : a;
             }),
           },