feat(frontend): add book/chapter/scene create buttons in project tree

[DankerMu]

Summary

• Add inline "+ 新建卷" / "+ 新建章节" / "+ 新建场景" buttons in the sidebar project tree
• Click button → inline input appears → Enter to confirm, Escape to cancel
• Tree auto-refreshes via TanStack Query invalidation after successful creation
• Disabled input during pending mutation to prevent double-submit

Test plan

• Click "+ 新建卷", type title, press Enter → new book appears in tree
• Click "+ 新建章节" under a book, type title, press Enter → new chapter appears
• Click "+ 新建场景" under a chapter, type title, press Enter → new scene appears
• Press Escape while typing → input dismissed, no creation
• Click away (blur) with empty input → input dismissed
• Verify pnpm build passes with no type errors

Closes #49

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