feat(projects): pin a thinking level next to a project's model

A project could pin the model new chats start on, but not the level to
run it at: the default cascade dropped any variant as soon as a project
model won, and only ever considered the global one — which belongs to
the global model.

Projects now carry `defaultVariant` alongside `defaultModel`, stored and
sanitized only next to that model, and the cascade passes it through.
Both controls sit in one "Defaults for new chats" group laid out like the
Sessions defaults, and the level appears only for models that offer them.

packages/ui/src/lib/i18n/messages/es.settings.ts

@@ -1132,9 +1132,12 @@ export const settingsDict = {
   "settings.projects.page.title.default": "Configuración del proyecto",
   "settings.projects.page.section.worktree": "Worktree",
   "settings.projects.page.field.projectName": "Nombre del proyecto",
+  "settings.projects.page.field.projectModel": "Modelo del proyecto",
+  "settings.projects.page.field.projectThinking": "Razonamiento del proyecto",
+  "settings.projects.page.section.chatDefaults": "Valores por defecto para chats nuevos",
+  "settings.projects.page.section.chatDefaultsDescription": "Se usa al iniciar un chat nuevo en este proyecto. Si está vacío, se aplican los valores globales. El razonamiento solo aparece en modelos que ofrecen niveles.",
   "settings.projects.page.field.projectNamePlaceholder": "Nombre del proyecto",
-  "settings.projects.page.field.defaultModel": "Modelo predeterminado para chats nuevos",
-  "settings.projects.page.field.defaultModelDescription": "Se usa al iniciar un chat nuevo en este proyecto. Si no se define, se usan los valores globales.",
+  "settings.projects.page.option.thinkingDefault": "El del modelo",
   "settings.projects.page.field.accentColor": "Color de énfasis",
   "settings.projects.page.field.projectIcon": "Icono del proyecto",
   "settings.projects.page.field.projectIconBackgroundAria": "Color de fondo del icono del proyecto",