Add 心经 as a second planted book, with a real bookshelf

[cubxxw]

Summary

• 我的树 was hardcoded to a single book (道德经). Generalized it into a Book-driven page at /tree/:bookId, with the data model and per-book content moved to src/books/ (types.ts, daodejing.ts, xinjing.ts, index.ts).
• Built the 首页书架 (Bookshelf) timeline page at / (src/Bookshelf.tsx / .css), a port of the 老树 · 首页书架.dc.html prototype, so books can actually be walked into instead of 我的树 only existing as an unreachable route.
• Added 《心经》(the Heart Sutra) as a second, complete, planted book — a Buddhism-themed counterpart to 道德经: five annotated branches (色空 / 五蕴 / 空相 / 无碍 / 揭谛), one 朱砂 (入木) leaf and one 青绿 (青芽) leaf with their own annotations, a thread continuation, and its own 周年回声. It's registered on the shelf timeline (year 649, Xuanzang's translation) alongside 道德经; 论语 / 庄子 remain as 即将种下 placeholders (PLACEHOLDER_BOOKS).
• Added react-router-dom for the / and /tree/:bookId routes.

Test plan

• npm run build (type-check + production build) passes
• Verified in a headless browser: / renders the timeline with 道德经, 心经 (live) and 论语, 庄子 (dormant placeholders); clicking a live plank navigates to /tree/:bookId
• Verified /tree/xinjing renders 心经's five branches, hover reveals the correct source/annotation/signature per branch, and its own 周年回声
• Verified /tree/lunyu (a placeholder with no tree yet) shows a "这本书还没有种下" state with a link back to the shelf instead of a blank/broken tree

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Generated by Claude Code

[cubxxw]

Superseded by #10.

This PR was the most ambitious of the batch — it introduced src/books/{types,index,daodejing,xinjing}.ts and a real 千年时间轴 Bookshelf.tsx. PR #1 crystallized the same idea into main first, but with a richer Book type (adds chapter: ReadingChapter for the /read page, plus StrataLayer and CompareSpan for sentence-level annotation strata) and a slightly different shelf implementation under src/pages/Bookshelf.tsx. The refactor plumbing here is now duplicative.

What was salvaged into #10: the intent to plant 心經 as a second 佛家 book. The tree/branch data here has been superseded by the Book shape on main; #10's xinjing.ts uses main's schema and draws the 心經 biography/reading content from #6 (which was written specifically for the biography page).

Closing in favor of #10. This PR's timeline design was 90% of the reason I trusted the "取之精华" approach — thanks.