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/formatters.test.ts

@@ -0,0 +1,45 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { formatRelativeDate } from './formatters';
+
+test('formatRelativeDate: future timestamps return "just now"', () => {
+  assert.equal(formatRelativeDate(Date.now() + 60_000), 'just now');
+});
+
+test('formatRelativeDate: 0ms ago returns "just now"', () => {
+  assert.equal(formatRelativeDate(Date.now()), 'just now');
+});
+
+test('formatRelativeDate: 30s ago returns "just now"', () => {
+  assert.equal(formatRelativeDate(Date.now() - 30_000), 'just now');
+});
+
+test('formatRelativeDate: 5 minutes ago returns "5m ago"', () => {
+  assert.equal(formatRelativeDate(Date.now() - 5 * 60_000), '5m ago');
+});
+
+test('formatRelativeDate: 59 minutes ago returns "59m ago"', () => {
+  assert.equal(formatRelativeDate(Date.now() - 59 * 60_000), '59m ago');
+});
+
+test('formatRelativeDate: 2 hours ago returns "2h ago"', () => {
+  assert.equal(formatRelativeDate(Date.now() - 2 * 3_600_000), '2h ago');
+});
+
+test('formatRelativeDate: 23 hours ago returns "23h ago"', () => {
+  assert.equal(formatRelativeDate(Date.now() - 23 * 3_600_000), '23h ago');
+});
+
+test('formatRelativeDate: 36 hours ago returns "Yesterday"', () => {
+  assert.equal(formatRelativeDate(Date.now() - 36 * 3_600_000), 'Yesterday');
+});
+
+test('formatRelativeDate: 5 days ago returns "5d ago"', () => {
+  assert.equal(formatRelativeDate(Date.now() - 5 * 86_400_000), '5d ago');
+});
+
+test('formatRelativeDate: 10 days ago returns locale date string', () => {
+  const ts = Date.now() - 10 * 86_400_000;
+  assert.equal(formatRelativeDate(ts), new Date(ts).toLocaleDateString());
+});