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.md

@@ -12,7 +12,7 @@ Every edit lands as `settings.mutate` path ops against the stored section — a
 
 ## Model list and endpoint interrogation
 
-A pi-ai profile's `models` list is edited on the card: one row per model showing its id and display name, with the context window and output cap behind a per-row disclosure and two label-free actions — expand and delete — on the right. An empty list means "serve this route's built-in catalog", so a row is only ever added deliberately; clearing a capacity drops it rather than storing a value the schema would reject, and the adapter's route-level fallbacks size whatever configuration leaves out. A capacity that is not a positive integer is simply not stored.
+A pi-ai profile's `models` list is edited on the card: one row per model showing its id and display name, with the context window and output cap behind a per-row disclosure and two label-free actions — expand and delete — on the right. An empty list means "serve this route's built-in catalog", so a row is only ever added deliberately; clearing a capacity drops it rather than storing a value the schema would reject, and the adapter's route-level fallbacks size whatever configuration leaves out — an empty capacity shows those fallbacks' magnitude as its placeholder, a hint rather than a mirror, since the field counts `K` as 1000 and a deployment may override them. A capacity that is not a positive integer is simply not stored.
 
 **Fetch available models** asks `llm.discoverModels` about the endpoint the form **currently shows**, including a base URL edited but not yet saved and a key typed but not yet stored, so adding a provider is one pass instead of save-then-return. The reply opens a picker rather than being written: candidates already configured start unchecked, so adopting a selection never overwrites a capacity the user corrected. A provider that cannot be interrogated is a detour, not a dead end — the adapter's own message appears beside the rows, which stay editable by hand.