[AI Config] Simple / Pro toggle and placeholder

[ndoschek]

PO: @ndoschek

The overhaul introduces a Simple mode for minimum setup and a Pro mode for the full surface of AI configuration possibilities. The actual Simple-mode contents come from the onboarding concept in #278. This ticket delivers the mode switcher and the Simple-mode container as a placeholder so downstream onboarding work has a fixed target to design into.

Acceptance scenario

Scenario: Switch between Simple and Pro modes
  Given the user opens the AI Configuration view
  When the user picks Simple mode via the toggle
  Then the Simple-mode page is shown (placeholder in this ticket)
  When the user switches back to Pro
  Then the full category tree and detail pages are shown again

Scope

• Simple / Pro toggle in the toolbar (rendered by T2 ([AI Config] New view structure and first port #311) next to the scope tabs). Uses the same switch element as the agent enable toggle if feasible.
• Mode selection remembered per scope. Pro becomes the default once a scope already has non-placeholder AI configuration.
• Simple-mode page: a single-page category shell with a placeholder body. Hosted by the abstract single-page primitive from T2.

Out of scope

• Actual Simple-mode page contents. Follow-up: #-tbd
• Any onboarding flow beyond the container.