chore: add HTMX TypeScript types and book picker implementation plan

- Add htmx.d.ts with TypeScript type definitions for HTMX global
- Add BOOK_PICKER_IMPL.md with implementation plan for book picker modal

web/src/types/htmx.d.ts

@@ -0,0 +1,4 @@
+declare var htmx: {
+  trigger(elt: Element | string, event: string, detail?: any): void;
+  find(selector: string): Element | null;
+};