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/en.settings.ts

@@ -1164,9 +1164,12 @@ export const settingsDict = {
   'settings.projects.page.title.default': 'Project Settings',
   'settings.projects.page.section.worktree': 'Worktree',
   'settings.projects.page.field.projectName': 'Project Name',
+  'settings.projects.page.field.projectModel': 'Project Model',
+  'settings.projects.page.field.projectThinking': 'Project Thinking',
+  'settings.projects.page.section.chatDefaults': 'Defaults for new chats',
+  'settings.projects.page.section.chatDefaultsDescription': 'Used when starting a new chat in this project. Falls back to the global defaults when unset. Thinking appears only for models that offer levels.',
   'settings.projects.page.field.projectNamePlaceholder': 'Project name',
-  'settings.projects.page.field.defaultModel': 'Default model for new chats',
-  'settings.projects.page.field.defaultModelDescription': 'Used when starting a new chat in this project. Falls back to global defaults when unset.',
+  'settings.projects.page.option.thinkingDefault': 'Model default',
   'settings.projects.page.field.accentColor': 'Accent Color',
   'settings.projects.page.field.projectIcon': 'Project Icon',
   'settings.projects.page.field.projectIconBackgroundAria': 'Project icon background color',