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

[cstns]

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

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