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/app/globals.css

@@ -71,6 +71,14 @@
   --color-popover: var(--popover);
   --color-card-foreground: var(--card-foreground);
   --color-card: var(--card);
+  /* The one sanctioned step below `text-xs` (12px). Minted so badges,
+     tag chips, and tabular-nums micro-labels stop hand-rolling
+     `text-[0.6875rem]` / `text-[0.65rem]` arbitrary values that drift by a
+     third of a pixel per site. UI-STANDARDS §5 pins the usage: never for
+     muted body or meta text (the muted floor stays `text-xs`) — only for
+     chips, badges, and tabular micro-labels with verified contrast. */
+  --text-2xs: 0.6875rem;
+  --text-2xs--line-height: 1rem;
   --radius-sm: calc(var(--radius) - 4px);
   --radius-md: calc(var(--radius) - 2px);
   --radius-lg: var(--radius);