fix(ui): AvatarStack — center inner Avatar in each tile (off-center clipping)

Each AvatarTile rendered at `size × size` with a 2px border under
`box-sizing: border-box`, giving a content area of `(size − 4) × (size − 4)`.
The inner `<Avatar size={size}>` exceeded the padding box, and the
`overflow-hidden + rounded-full` clip — centered on the wrapper — combined
with the avatar contents anchored at the padding-edge top-left to displace
photos and especially the centered initials gradient + letter toward the
lower-right of the visible disc, leaving a sliver of background opposite.
Compounding this, `Avatar`'s `inline-flex` root sat on the line-box text
baseline, so any inherited `line-height ≠ 1` (the DM list inherits the
row's line-height) drifted the avatar a further several px vertically.

Two corrections:
  • Size the inner Avatar to `size − 2·TILE_BORDER_WIDTH` (matches the
    padding box) — extracted as a constant so the dependency between
    `border-2` and the inner size is visible.
  • Center geometrically via `flex items-center justify-center` on the
    wrapper, bypassing inline-flow placement so the Avatar is anchored
    regardless of inherited type metrics.

Verified in the live app (DM sidebar, chat header, welcome header) and
with a 4-tile diamond at sizes 32 and 80: visible offset is now exactly
the border width on every tile, letters/photos sit dead-center.

Adds a regression test that pins both invariants (flex centering classes
present + inner Avatar style.width === tileSize − 4) so a future change
that re-introduces the bug fails fast. 12/12 AvatarStack tests, 365/365
web tests, typecheck clean.

design-system.md spec updated with the AvatarTile geometry contract.

packages/web/src/components/ui/AvatarStack.test.tsx

@@ -164,6 +164,35 @@ describe('AvatarStack', () => {
     expect(overflowTop).toBeGreaterThan(Math.max(...tileTops) - 1);
   });
 
+  it('sizes the inner Avatar to the tile content area so contents stay centered', () => {
+    // Regression for the off-center clipping bug: when the inner Avatar was
+    // sized to the outer tile dimensions, `box-sizing: border-box` + 2px border
+    // pushed its contents (image crop, initials gradient + letter) toward the
+    // lower-right of the visible disc. The fix is to size the inner Avatar to
+    // (tileSize − 2·border) and center it geometrically with flex.
+    const { container } = render(
+      <AvatarStack members={makeUsers(4)} size={80} border="chat" />,
+    );
+    const tiles = Array.from(
+      container.querySelectorAll('[data-avatar-stack-tile]'),
+    ) as HTMLElement[];
+    expect(tiles.length).toBe(4);
+    for (const tile of tiles) {
+      // Tile is centered as a flex container so the Avatar bypasses inline-flow
+      // baseline drift entirely.
+      expect(tile.className).toMatch(/\bflex\b/);
+      expect(tile.className).toMatch(/items-center/);
+      expect(tile.className).toMatch(/justify-center/);
+      // Inner Avatar is sized to (tileSize − 2·border) so it fits the padding
+      // box exactly. With size=80 and tileRatio=0.58, tileSize=46 → inner=42.
+      const tileSize = parseInt(tile.style.width, 10);
+      const inner = tile.querySelector('[data-avatar]') as HTMLElement | null;
+      expect(inner).toBeTruthy();
+      expect(inner!.style.width).toBe(`${tileSize - 4}px`);
+      expect(inner!.style.height).toBe(`${tileSize - 4}px`);
+    }
+  });
+
   it('renders icon override and ignores the stack', () => {
     const { container } = render(
       <AvatarStack