[SDUI][P2] App & Utility page types (pageType app/utility)

[xuyushun441-sys]

Context

pageType: 'app' and pageType: 'utility' are defined in the schema but not implemented — no synthesis, no assignment, no renderer wiring. Cross-page navigation is not schema-driven.

Goal

Support app-level and utility pages as assignable, schema-rendered surfaces, enabling schema-driven cross-page navigation.

Scope

• Assignment + render wiring for pageType: 'app' and 'utility'.
• Minimal default synthesis or blank-canvas authoring.
• Hook into the (schema-driven) navigation tree so these pages are reachable.

Acceptance

• An authored app/utility page renders via SchemaRenderer
• Reachable from schema-driven navigation
• Editable via PageDesigner (depends on page persistence)