fix(ui-models): restore the add-provider row and hint an empty capacity

The two ways to gain a provider had picked up the shared button base's pill
shape and shrunk to their labels, so they read as two stray buttons of
different lengths under the list instead of its last slot. They split the row
evenly again, on the row cards' own corner and the dashed outline this page
already uses for "nothing here yet"; the rule that overrides the base now
says so in one place rather than layering a second `.addButton` block.

An empty capacity shows the adapter's route-level fallback as its placeholder,
so a blank field reads as "sized by the route" rather than as a model with no
capacity. It is a hint, not a mirror: the field counts K as 1000 while the
fallback is 262144, and a deployment may override it.

The picker's description says what the list is without promising an edit the
rows themselves already offer.

packages/client/ui-models/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-models/README.md
-README.md: 9d9833cf269d6b2225605e5a4b095d291d97a6bd
-README.zh.md: c19499a1e49a8d537a1722ab4dc4c4b6f0ee2026
+README.md: b55914197e472edec8a8b6d4d3e02036d1697728
+README.zh.md: ca93c3d5a2a85fffb22707f8389f1e979468e2ec