jbot: fix no-scroll viewport layout

- index.css targeted #root but app mounts to #jbot-root — viewport
  height was never constrained, causing full page scroll
- Fix: rename #root → #jbot-root, add html/body {height:100%;overflow:hidden}
- .app now uses height:100% relative to #jbot-root instead of
  duplicate 100dvh, so the layout is a true single-screen SPA

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

static_src/jbot/App.css

@@ -13,8 +13,7 @@
 
 /* ── App shell ────────────────────────────────────────────── */
 .app {
-  height: 100dvh;
-  height: -webkit-fill-available;
+  height: 100%;
   display: flex;
   flex-direction: column;
   background: var(--bg);