feat(nav): expose current slide frontmatter

[NgoQuocViet2001]

Description

Global layer components cannot use $frontmatter because they render outside an individual slide. They can already use the reactive $nav.currentLayout, but arbitrary active-slide frontmatter is not exposed through the same API.

This change:

• exposes $nav.currentFrontmatter and useNav().currentFrontmatter
• keeps the value reactive as navigation changes the current slide
• documents direct and composable usage
• adds focused coverage for the initial and next slide frontmatter

Closes #2659

Validation

• pnpm build
• pnpm test — 26 files, 219 tests passed
• pnpm exec vitest run --project @slidev/client — 3 files, 4 tests passed
• pnpm exec eslint packages/client/composables/useNav.ts packages/client/composables/useNav.test.ts
• git diff --check

Local baseline limitations

• pnpm typecheck reaches unrelated existing errors in cypress.config.ts, VitePress declaration resolution, and duplicate markdown-exit versions; it reports no error in the changed files.
• Full pnpm lint on Windows reports repository-wide CRLF formatting differences. The changed TypeScript files pass the scoped lint command above.

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commit: 650cec2