feat(chats): replace book filter pills with a dropdown selector

[yicheng47]

Summary

Implements #278, plus two chat-rendering fixes folded in per review discussion.

Chats book filter dropdown (#278)

• Replace the book-filter pill row with a compact dropdown next to the search input: "All Books" with total count, then one entry per book with its chat count, sorted by count descending
• Select primitive gains an optional right-aligned detail per option (backward-compatible with all existing consumers)
• Sort buttons (Newest/Oldest) move onto the search line; the chat list gains the reclaimed vertical space
• No visible horizontal or vertical scrollbars on the Chats page (scrollbar-none on the list; the only overflow-x-auto element was the pill row, now gone)
• No new i18n strings needed — reuses chats.allBooks

Render GFM markdown in AI replies

AI replies containing tables rendered as literal pipe text because bare react-markdown is CommonMark-only. Added remark-gfm at all six <Markdown> call sites (MessageBubble, LookupPopover, ExplainPopover, VocabDetailModal), so tables, strikethrough, and task lists now render.

Markdown style extraction

MessageBubble's inline markdown class wall (including inert prose classes — @tailwindcss/typography was never installed) moved to a .markdown-body class in index.css, a 1:1 translation using the design tokens, extended with table styles (bordered cells, semibold headers, horizontal scroll for wide tables in the narrow AI panel).

Closes #278

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