feat(customization): PageBuilder runtime for application-specific auth pages

[windischb]

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What

Renders Realm- and Application-authored PageBuilder schemas at runtime for the login, password-forgot, and logout auth pages, plus the Application Settings → Pages UI, page inheritance (Realm → Application), and public effective schemas in app-info.

Built against the @cocoar/vue-page-builder v2.17 contract; all @cocoar/vue-* packages bumped to 2.17.1.

Feature flag — safe to merge

Everything is gated behind AppSettings.Features.PageBuilder (operator-level, defaults to false). While off: editor, API, app-info, and runtime schemas are all masked; stored data is retained. Merging to develop ships it dark.

Non-obvious behavior

• Application page schemas are separately managed and intentionally absent from ApplicationSettingsDto — regular settings replacement preserves ApplicationSettings.Pages (pinned by an integration test).
• Application DELETE /pages/{slug} = "inherit the Realm slot"; Realm DELETE = "use the fixed host fallback".
• Tenant schemas may invoke only host-owned allowlisted actions/elements. MFA choice, TOTP/email-OTP, secure setup, and passwordless-policy handling stay in fixed UI. Invalid/unavailable schemas and ?safemode=1 fall back to the fixed view.
• Local/federated sign-out lands on /logged-out so the logout slot has a real runtime surface.

Verified locally (Chrome DevTools)

• Flag on → app-info exposes Features.PageBuilder:true; editor visible under /platform/customization/pages.
• Full round-trip: author schema → save → persisted (app-info + ANGEPASST badge) → runtime render on /login.
• Wrong credentials on the schema-rendered login → error banner surfaces.
• Functional login through the schema form → dashboard.
• Responsive at desktop + mobile (390px); ?safemode=1 → fixed fallback.
• Built a split-screen layout (Stack direction: row) to confirm non-trivial layouts render.
• Reset-to-default persists (Pages:{}); no console errors.

Test coverage

• Modgud.Tests.Unit/Api/Features/Admin/AppSettingsEndpointsTests.cs
• Modgud.Tests.Unit/Applications/EffectiveSettingsTests.cs
• Modgud.Api.Tests/Authorization/PageBuilderFeatureFlagTests.cs

Docs: docs/platform/pages.md.

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