feat(e10): put the 1024x768 size contract inside the primitives

S1c, the last part of S1. The contract now lives in components/ui/ and
in one CSS recipe instead of in ~160 call sites:

- Minimum target 2.75rem, on coarse pointers only (.touch-target). A
  32px icon button is fine with a mouse -- precise pointer, hover to
  confirm the aim -- and not fine with a thumb, so it grows exactly where
  there is no precise pointer, which on this app's real second device is
  the tablet. It grows the real box rather than painting an invisible
  ::after hit area, because two expanded hit areas on icon buttons 4px
  apart would overlap and the overlap fires the WRONG button. `link` is
  excluded: it is text inside a sentence.
- 2.75rem rather than 44px so the target obeys the text-size slider like
  everything else; verified that the type/geometry ladder was already
  rem-derived off --font-scale, so nothing else was needed there.
- The ladder is documented as a closed set in Button.tsx, with the site
  counts it was derived from, and ui/README.md states the contract.

Measured in the browser: fine pointer 36x36 and 34px-tall buttons
(unchanged), coarse pointer the same buttons at 44x44 and 44px tall.
1024x768 renders identically. 3x tsc, eslint, knip, prettier and 690
vitest green.

frontend/src/components/ui/Button.dom.test.tsx

@@ -98,6 +98,18 @@ describe("Button primitive (S5)", () => {
     }
   });
 
+  it("carries the coarse-pointer minimum target, except on a link", () => {
+    // The 1024x768 comfort contract: the minimum lives in the primitive, so no call site can opt
+    // out of it by accident. A link is text in a sentence and is excluded by design.
+    const { unmount } = render(<Button>Tap</Button>);
+    expect(screen.getByRole("button", { name: "Tap" }).className).toContain("touch-target");
+    unmount();
+    render(<Button variant="link">Read more</Button>);
+    expect(screen.getByRole("button", { name: "Read more" }).className).not.toContain(
+      "touch-target",
+    );
+  });
+
   it("fullWidth adds the w-full utility", () => {
     render(<Button fullWidth>Wide</Button>);
     expect(screen.getByRole("button", { name: "Wide" }).className).toContain("w-full");