perf: bound the renderer-side VOD storyboard cache (FIFO 100)

vodStoryboardClientCache was a plain Map<vodId, VodStoryboard | null>
with no eviction. Every VOD ever hovered cached its first sprite
data URL — about 50-200 KB each. Browsing a long-running streamer's
2000-VOD archive could leave the renderer holding 100-400 MB of
hover-only sprite data permanently, with no signal to the user
that it was happening.

Wrapped writes in a rememberStoryboard helper that caps the cache
at 100 entries with FIFO eviction (Map iterator is insertion-ordered
so .keys().next().value is always the oldest). Cache hit / miss
semantics unchanged for the live set — only the dropped-off-the-
back entries get re-fetched if the user scrolls back to a VOD they
hovered hundreds of cards ago, and that re-fetch is fast because
the main-process side has its own metadata cache that survives the
renderer-side eviction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/renderer-vod-hover.ts

@@ -21,6 +21,20 @@ let pendingHoverVodId: string | null = null;
 const HOVER_DEBOUNCE_MS = 220;
 const FRAME_INTERVAL_MS = 600;
 const FRAMES_TO_CYCLE = 4;
+// Bounded cache — each storyboard data URL is ~50-200 KB, so an
+// unbounded cache could balloon to hundreds of MB on a long browsing
+// session through a streamer with thousands of VODs. FIFO eviction
+// keeps the working set fresh without manual cleanup.
+const MAX_CLIENT_STORYBOARD_CACHE = 100;
+
+function rememberStoryboard(vodId: string, sb: VodStoryboard | null): void {
+    vodStoryboardClientCache.set(vodId, sb);
+    if (vodStoryboardClientCache.size > MAX_CLIENT_STORYBOARD_CACHE) {
+        // Map iterator is insertion-ordered — first key is the oldest.
+        const oldestKey = vodStoryboardClientCache.keys().next().value as string | undefined;
+        if (oldestKey !== undefined) vodStoryboardClientCache.delete(oldestKey);
+    }
+}
 
 function ensureVodHoverHandlersBound(): void {
     const grid = document.getElementById('vodGrid');
@@ -85,7 +99,7 @@ async function activateHoverPreview(card: HTMLElement, vodId: string): Promise<v
         } catch (_) {
             storyboard = null;
         }
-        vodStoryboardClientCache.set(vodId, storyboard);
+        rememberStoryboard(vodId, storyboard);
     }
 
     // Cursor may have moved on while we awaited; re-check guard.