feat(webapp): auto-select and optimize project selector

Automatically set the Vercel project when entering project-selection if
there is at least one available project and none selected. This prevents
an empty selection state and reduces manual clicks for the common case
of a single or first project.

Disable the Select when there is exactly one available project to make
the intent explicit and avoid unnecessary interaction. Add a filter
prop to the Select only when there are more than five projects to keep
the dropdown performant and avoid enabling filtering for small lists.

Adjust the effect deps to trigger selection only on relevant state and
project list changes.

Author: 0ski

apps/webapp/app/components/integrations/VercelOnboardingModal.tsx

@@ -565,6 +565,12 @@ export function VercelOnboardingModal({
     }
   }, [state, customEnvironments, vercelStagingEnvironment]);
 
+  useEffect(() => {
+    if (state === "project-selection" && availableProjects.length > 0 && !selectedVercelProject) {
+      setSelectedVercelProject(availableProjects[0]);
+    }
+  }, [state, availableProjects, selectedVercelProject]);
+
   if (!isOpen || onboardingData?.authInvalid) {
     return null;
   }
@@ -625,6 +631,7 @@ export function VercelOnboardingModal({
                 </Callout>
               ) : (
                 <Select
+                  disabled={availableProjects.length === 1}
                   value={selectedVercelProject?.id || ""}
                   setValue={(value) => {
                     if (!Array.isArray(value)) {
@@ -634,6 +641,7 @@ export function VercelOnboardingModal({
                     }
                   }}
                   items={availableProjects}
+                  filter={availableProjects.length > 5 ? { keys: ["name"] } : undefined}
                   variant="tertiary/medium"
                   placeholder="Select a Vercel project"
                   dropdownIcon