Fix #28: stash live stats for beforeunload so leaderboard gets real score

The beforeunload fallback in shell.html sends session-end data when the
user closes the tab. Previously Module._analyticsLastStats was only set
inside doEnd(), which runs when C explicitly ends a session. For the
very common case of a player closing the tab mid-game (first session),
the field was null and the fallback sent score 0.

Add analytics_stash_stats() which serialises current stats into
_analyticsLastStats every frame, so the beforeunload handler always
has up-to-date data regardless of when the tab is closed.

src/game/analytics.c

@@ -167,6 +167,20 @@ EM_JS(void, js_analytics_send_end, (int score, int level_reached,
     doEnd(sid, endReason, score, level_reached, lives_used, duration_secs);
 });
 
+/* Stash current stats as JSON so the beforeunload fallback in
+ * shell.html has up-to-date data if the user closes the tab. */
+EM_JS(void, js_analytics_stash_stats, (int score, int level_reached,
+                                        int lives_used, int duration_secs), {
+    if (!Module._analyticsUrl) return;
+    Module._analyticsLastStats = JSON.stringify({
+        score: score,
+        level_reached: level_reached > 0 ? level_reached : 1,
+        lives_used: lives_used,
+        duration_seconds: duration_secs,
+        end_reason: 'quit'
+    });
+});
+
 /* ── C wrappers ─────────────────────────────────────────────────── */
 
 void analytics_init(void) {
@@ -188,6 +202,16 @@ void analytics_session_end(GameStats *stats, const char *end_reason) {
     );
 }
 
+void analytics_stash_stats(GameStats *stats) {
+    stats_update_score(stats);
+    js_analytics_stash_stats(
+        stats->score,
+        stats->levels_completed > 0 ? stats->levels_completed : 1,
+        stats->deaths,
+        (int)stats->time_elapsed
+    );
+}
+
 #else
 /* ── Non-WASM stubs ─────────────────────────────────────────────── */
 
@@ -202,4 +226,8 @@ void analytics_session_end(GameStats *stats, const char *end_reason) {
     (void)end_reason;
 }
 
+void analytics_stash_stats(GameStats *stats) {
+    (void)stats;
+}
+
 #endif /* __EMSCRIPTEN__ */

src/game/analytics.h

@@ -15,4 +15,8 @@ void analytics_session_start(void);
  * end_reason: "death", "quit", "timeout", or "completed". */
 void analytics_session_end(GameStats *stats, const char *end_reason);
 
+/* Stash current stats so the beforeunload fallback has up-to-date data
+ * if the user closes the tab mid-session. Call periodically. */
+void analytics_stash_stats(GameStats *stats);
+
 #endif /* JNR_ANALYTICS_H */

src/main.c

@@ -578,9 +578,10 @@ static void game_update(float dt) {
 
     level_update(&s_level, dt);
 
-    /* Accumulate play time */
+    /* Accumulate play time and keep the beforeunload fallback current. */
     if (s_session_active) {
         s_stats.time_elapsed += dt;
+        analytics_stash_stats(&s_stats);
     }
 
     /* Check for level exit transition */