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/web/server/lib/opencode/settings-normalization-runtime.js

@@ -155,6 +155,7 @@ export const createSettingsNormalizationRuntime = (dependencies) => {
       const iconBackground = normalizeIconBackground(candidate.iconBackground);
       const color = typeof candidate.color === 'string' ? candidate.color.trim() : '';
       const defaultModel = typeof candidate.defaultModel === 'string' ? candidate.defaultModel.trim() : '';
+      const defaultVariant = typeof candidate.defaultVariant === 'string' ? candidate.defaultVariant.trim() : '';
       const addedAt = Number.isFinite(candidate.addedAt) ? Number(candidate.addedAt) : null;
       const lastOpenedAt = Number.isFinite(candidate.lastOpenedAt)
         ? Number(candidate.lastOpenedAt)
@@ -175,6 +176,8 @@ export const createSettingsNormalizationRuntime = (dependencies) => {
         ...(iconBackground ? { iconBackground } : {}),
         ...(color ? { color } : {}),
         ...(defaultModel && defaultModel.includes('/') ? { defaultModel } : {}),
+        // A variant is meaningless without the model it belongs to.
+        ...(defaultModel && defaultModel.includes('/') && defaultVariant ? { defaultVariant } : {}),
         ...(Number.isFinite(addedAt) && addedAt >= 0 ? { addedAt } : {}),
         ...(Number.isFinite(lastOpenedAt) && lastOpenedAt >= 0 ? { lastOpenedAt } : {}),
       };