refactor(ui): call a book a project

A book is the use case Margin was built for, not the limit of it. Blog posts published straight to
the web are coming, and "New book" on the shelf is the wrong invitation for that. Project covers
both without promising anything the app does not do.

This is user-visible strings only. Internal identifiers keep saying book, because renaming bookId
and setBooks is churn nobody reads.

Claude-Session: https://claude.ai/code/session_018HwEWvoE1NkkUjJCMLSnup

src-tauri/src/lib.rs

@@ -18,7 +18,7 @@ use tauri::{Emitter, Runtime};
 fn build_menu<R: Runtime>(handle: &tauri::AppHandle<R>) -> tauri::Result<Menu<R>> {
     let menu = Menu::default(handle)?;
 
-    let new_book = MenuItemBuilder::with_id("new-book", "New Book")
+    let new_book = MenuItemBuilder::with_id("new-book", "New Project")
         .accelerator("CmdOrCtrl+N")
         .build(handle)?;
     let save = MenuItemBuilder::with_id("save", "Save")

src-tauri/src/library.rs

@@ -16,7 +16,7 @@ pub struct BookSummary {
 fn corrupt_summary(stem: &str) -> BookSummary {
     BookSummary {
         id: stem.to_string(),
-        title: "Unreadable book".to_string(),
+        title: "Unreadable project".to_string(),
         author: String::new(),
         corrupt: true,
         updated_at: 0,