Redirect unauthorized users from application creation and align `/check-name` behavior

### Description

**Context / Problem**
Users without team-level permission to create an application can access the application creation form. They should be redirected instead. Additionally, the `/check-name` endpoint must consider both team and application contexts when checking instance names during application creation to avoid false negatives for users lacking proper rights.

**Acceptance criteria**
- Users without team permissions to create an application are redirected when accessing the application creation form.
- Direct URL access to the form results in a redirect to an authorized location with an appropriate message.
- Front end prevents navigation to the application creation form if permissions are missing.
- `/check-name` verifies both team and application context when invoked during creation.
- Tests cover redirect logic, correct authorization responses, and accurate `/check-name` validation for authorized vs unauthorized users.

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate