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/insights/metric-target-summary.tsx

@@ -354,7 +354,7 @@ function TargetReferencePanel({
           }
         />
         {heading ? (
-          <p className="text-muted-foreground text-[0.6875rem] font-medium tracking-[0.06em] uppercase">
+          <p className="text-muted-foreground text-xs font-medium tracking-[0.06em] uppercase">
             {heading}
           </p>
         ) : null}