feat(composition): add page and settings primitives

[ditto-m]

Problem

• Fleet apps lack a shared page measure and restrained title hierarchy for content-heavy and utility routes.
• Settings screens compose controls inconsistently across mobile and desktop, including accessible associations between row copy and controls.

Approach

• Add PageContainer and PageHeader primitives with narrow, regular, and wide measures plus compact and regular title density.
• Add SettingsGroup and SettingsRow primitives for divided settings rows, explicit heading hierarchy, accessible control labels, and a visually separate danger zone.
• Add typography tokens and utility classes for app-owned content that needs the same hierarchy.
• Keep product cards, campaign imagery, charts, and control selection app-owned instead of introducing a monolithic settings-page component.
• Add showcase examples and responsive Playwright coverage for phone, tablet, and desktop layouts.

Review notes

• Version advances to 0.49.0 because the change adds public exports without removing existing APIs.
• Validated on the current-main merge tree with 140 unit tests, TypeScript type checking, package and showcase production builds, and 21 Playwright checks.
• Completed two adversarial review rounds covering public API stability, accessibility, responsive behavior, CSS interoperability, and regression risk.

Impact

• Consumers can adopt the new primitives incrementally; existing components and styles remain available.
• Theme selection remains app-owned. Light-only apps can pin the light theme without exposing a dark-mode control.

Rollback

• Revert the new exports, composition styles, showcase route, and 0.49.0 metadata before publishing the package.