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.zh.md

@@ -12,7 +12,7 @@
 
 ## 模型列表与端点询问
 
-pi-ai profile 的 `models` 列表就在卡片上编辑：一行一个模型，行上显示 id 与显示名称，上下文窗口与输出上限收在该行的展开区内，右侧是两个无文字的操作——展开与删除。空列表意味着「使用该路由的内置 catalog」，因此每一行都只会被刻意添加；清空容量会丢弃它，而不是存入一个 schema 会拒绝的值，配置留空的部分由适配器的路由级回退值定尺寸。不是正整数的容量根本不会被存下。
+pi-ai profile 的 `models` 列表就在卡片上编辑：一行一个模型，行上显示 id 与显示名称，上下文窗口与输出上限收在该行的展开区内，右侧是两个无文字的操作——展开与删除。空列表意味着「使用该路由的内置 catalog」，因此每一行都只会被刻意添加；清空容量会丢弃它，而不是存入一个 schema 会拒绝的值，配置留空的部分由适配器的路由级回退值定尺寸——留空的容量以这些回退值的量级作为占位符，那只是提示而非镜像：该字段按 1000 计 `K`，且部署可以覆盖这些回退值。不是正整数的容量根本不会被存下。
 
 **获取可用模型**会针对表单**当前显示**的端点调用 `llm.discoverModels`，包括已修改但尚未保存的 API 地址和已键入但尚未存储的密钥，因此新增一个提供方是一趟走完，而不是「先保存再回来」。回复会打开一个选择框而不是直接写入：已配置过的候选默认不勾选，因此采纳一次选择绝不会覆盖用户已更正的容量。无法被询问的提供方只是绕路而非死路——适配器自己的消息会显示在各行旁边，而这些行仍可手工编辑。