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/lib/chart-theme.ts

@@ -0,0 +1,8 @@
+export const CHART_THEME = {
+  tick: '#a5adcb',
+  tooltipBg: '#363a4f',
+  tooltipBorder: '#494d64',
+  tooltipText: '#cad3f5',
+  tooltipLabel: '#b8c0e0',
+  barFill: '#8aadf4',
+} as const;