refactor(ui): mint the text-2xs token and the neutral TagChip

Three integration cards carried the same hand-copied chip literal in the
card-header accessory slot, each free to drift on the next touch; the size
itself was an arbitrary value repeated at three slightly different spellings
across the tree.

- Mint `--text-2xs` (0.6875rem with a matched line-height) as the one
  sanctioned step below `text-xs`.
- Mint `TagChip` and adopt it on the Fitbit, Google Health, and Nightscout
  cards. It stays distinct from `Badge`, which carries status meaning.
- Sweep the arbitrary micro sizes: the stepper label and the two dose-ledger
  outline badges move onto the token; the muted target caption and the muted
  ledger meta row move up to the `text-xs` muted floor.

No visible change beyond the two muted rows gaining a fraction of a pixel.

src/components/ui/__tests__/tag-chip.test.tsx

@@ -0,0 +1,30 @@
+import { describe, expect, it } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+import { TagChip } from "../tag-chip";
+
+describe("<TagChip>", () => {
+  it("renders the neutral chip shape on the text-2xs token", () => {
+    const html = renderToStaticMarkup(<TagChip>Glucose</TagChip>);
+    expect(html).toContain("Glucose");
+    expect(html).toContain('data-slot="tag-chip"');
+    expect(html).toContain("bg-muted");
+    expect(html).toContain("text-foreground");
+    expect(html).toContain("rounded-full");
+    expect(html).toContain("text-2xs");
+    // The arbitrary value the three hand-copied sites carried is gone.
+    expect(html).not.toContain("text-[0.6875rem]");
+  });
+
+  it("merges caller classes and forwards span props", () => {
+    const html = renderToStaticMarkup(
+      <TagChip className="ml-2" title="source label">
+        Steps
+      </TagChip>,
+    );
+    expect(html).toContain("ml-2");
+    expect(html).toContain('title="source label"');
+    // The base shape survives the merge.
+    expect(html).toContain("rounded-full");
+  });
+});