Dashboard: every number leads somewhere, real counts, and new charts

- Cards and charts link to the page they're about: pending messages to the
  queue, bans to blocked IPs, report warnings to the reports, and so on,
  shown only to viewers who may open that page.
- A one-line status under the greeting: what needs a look (failed tasks,
  messages retrying, recipients given up on) or, when nothing does, what's
  there. Each phrase is a link.
- Counts from the server's own objects stand in for live metrics it can't
  report, and a live number with no source reads as unknown, not zero.
- Who uses the space: a treemap of people sized by storage, colored by how
  near their quota they are, each tile opening the account.
- Where mail is waiting: queued recipients by destination, split into
  waiting, retrying and given up, each row opening the filtered queue.
- The weekly rhythm: messages by hour and weekday, shown once metric
  history exists.
- Dashboard tabs are titled by their label.

src/features/dashboard/treemap.test.ts

@@ -0,0 +1,36 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { describe, expect, it } from 'vitest';
+import { squarify } from './treemap';
+
+describe('squarify', () => {
+  it('fills the box, with areas in proportion to the values', () => {
+    const tiles = squarify([6, 6, 4, 3, 2, 2, 1], (v) => v, 600, 400);
+    expect(tiles).toHaveLength(7);
+    const area = tiles.reduce((s, t) => s + t.w * t.h, 0);
+    expect(area).toBeCloseTo(600 * 400, 3);
+    for (const t of tiles) {
+      expect(t.w * t.h).toBeCloseTo((t.item / 24) * 600 * 400, 3);
+      expect(t.x).toBeGreaterThanOrEqual(-1e-9);
+      expect(t.y).toBeGreaterThanOrEqual(-1e-9);
+      expect(t.x + t.w).toBeLessThanOrEqual(600 + 1e-6);
+      expect(t.y + t.h).toBeLessThanOrEqual(400 + 1e-6);
+    }
+  });
+
+  it('keeps tiles reasonably square', () => {
+    const tiles = squarify([6, 6, 4, 3, 2, 2, 1], (v) => v, 600, 400);
+    const worst = Math.max(...tiles.map((t) => Math.max(t.w / t.h, t.h / t.w)));
+    expect(worst).toBeLessThan(4);
+  });
+
+  it('gives nothing for nothing', () => {
+    expect(squarify([0, 0], (v) => v, 100, 100)).toEqual([]);
+    expect(squarify([], (v: number) => v, 100, 100)).toEqual([]);
+    expect(squarify([1], (v) => v, 0, 100)).toEqual([]);
+  });
+});