feat(stats): build anime-centric stats dashboard frontend

5-tab React dashboard with Catppuccin Mocha theme:
- Overview: hero stats, streak calendar, watch time chart, recent sessions
- Anime: grid with cover art, episode list with completion %, detail view
- Trends: 15 charts across Activity, Efficiency, Anime, and Patterns
- Vocabulary: POS-filtered word/kanji lists with detail panels
- Sessions: expandable session history with event timeline

Features:
- Cross-tab navigation (anime <-> vocabulary)
- Global word detail panel overlay
- Expandable episode detail with Anki card links (Expression field)
- Per-anime multi-line trend charts
- Watch time by day-of-week and hour-of-day
- Collapsible sections with accessibility (aria-expanded)
- Card size selector for anime grid
- Cover art caching via AniList
- HTTP API client with file:// protocol fallback for Electron overlay

stats/src/components/anime/CollapsibleSection.tsx

@@ -0,0 +1,28 @@
+import { useId, useState } from 'react';
+
+interface CollapsibleSectionProps {
+  title: string;
+  defaultOpen?: boolean;
+  children: React.ReactNode;
+}
+
+export function CollapsibleSection({ title, defaultOpen = true, children }: CollapsibleSectionProps) {
+  const [open, setOpen] = useState(defaultOpen);
+  const contentId = useId();
+
+  return (
+    <div className="bg-ctp-surface0 border border-ctp-surface1 rounded-lg">
+      <button
+        type="button"
+        onClick={() => setOpen(!open)}
+        aria-expanded={open}
+        aria-controls={contentId}
+        className="w-full flex items-center justify-between p-4 text-left"
+      >
+        <h3 className="text-sm font-semibold text-ctp-text">{title}</h3>
+        <span className="text-ctp-overlay2 text-xs" aria-hidden="true">{open ? '▲' : '▼'}</span>
+      </button>
+      {open && <div id={contentId} className="px-4 pb-4">{children}</div>}
+    </div>
+  );
+}