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/overview/HeroStats.tsx

@@ -0,0 +1,51 @@
+import { StatCard } from '../layout/StatCard';
+import { formatDuration, formatNumber, todayLocalDay, localDayFromMs } from '../../lib/formatters';
+import type { OverviewSummary } from '../../lib/dashboard-data';
+import type { SessionSummary } from '../../types/stats';
+
+interface HeroStatsProps {
+  summary: OverviewSummary;
+  sessions: SessionSummary[];
+}
+
+export function HeroStats({ summary, sessions }: HeroStatsProps) {
+  const today = todayLocalDay();
+  const sessionsToday = sessions.filter(
+    (s) => localDayFromMs(s.startedAtMs) === today,
+  ).length;
+
+  return (
+    <div className="grid grid-cols-2 xl:grid-cols-6 gap-3">
+      <StatCard
+        label="Watch Time Today"
+        value={formatDuration(summary.todayActiveMs)}
+        color="text-ctp-blue"
+      />
+      <StatCard
+        label="Cards Mined Today"
+        value={formatNumber(summary.todayCards)}
+        color="text-ctp-green"
+      />
+      <StatCard
+        label="Sessions Today"
+        value={formatNumber(sessionsToday)}
+        color="text-ctp-lavender"
+      />
+      <StatCard
+        label="Episodes Today"
+        value={formatNumber(summary.episodesToday)}
+        color="text-ctp-teal"
+      />
+      <StatCard
+        label="Current Streak"
+        value={`${summary.streakDays}d`}
+        color="text-ctp-peach"
+      />
+      <StatCard
+        label="Active Anime"
+        value={formatNumber(summary.activeAnimeCount)}
+        color="text-ctp-mauve"
+      />
+    </div>
+  );
+}