refactor: move TodoMVC example into example/ directory

- example/todomvc/pocketbook/todomvc.cljs — app source
- example/todomvc/todomvc.html — page
- example/todomvc/js/ — compiled output (gitignored)

Server static file serving is now filesystem-based via --static-dir
flag instead of classpath resources. No static dir by default (API
only); bb server passes --static-dir example/todomvc.

Removed resources/public/ — library has no bundled static assets.

test/pocketbook/server_test.clj

@@ -61,14 +61,13 @@
 ;; Tests
 ;; ---------------------------------------------------------------------------
 
-(deftest serves-static-root
+(deftest root-returns-404-without-static-dir
   (let [req  (-> (HttpRequest/newBuilder)
                  (.uri (URI. (url "/")))
                  (.GET)
                  (.build))
         resp (.send client req (HttpResponse$BodyHandlers/ofString))]
-    (is (= 200 (.statusCode resp)))
-    (is (.contains (.body resp) "TodoMVC"))))
+    (is (= 404 (.statusCode resp)))))
 
 (deftest push-and-pull
   (testing "Push new documents"