fix: eliminate scroll stutter and fix breadcrumb shadow consistency

Replace React state (useState + re-render) with direct DOM manipulation
for the breadcrumb title visibility. IntersectionObserver now toggles a
data-title-hidden attribute on the bar element, and CSS handles shadow
+ title visibility. This avoids re-rendering the heavy BlockNote editor
on every scroll intersection change.

Fixes: shadow always appears when title is hidden (CSS-driven, no race),
scroll is smooth (zero React re-renders during scroll).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/Editor.css

@@ -23,6 +23,24 @@
   opacity: 0.55;
 }
 
+/* Breadcrumb bar: title + shadow toggled via data attribute (no React re-render) */
+.breadcrumb-bar {
+  transition: box-shadow 0.2s ease;
+  box-shadow: none;
+}
+
+.breadcrumb-bar[data-title-hidden] {
+  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.08);
+}
+
+.breadcrumb-bar__title {
+  display: none;
+}
+
+.breadcrumb-bar[data-title-hidden] .breadcrumb-bar__title {
+  display: flex;
+}
+
 /* Scroll area wrapping title + editor — single scroll context for alignment */
 .editor-scroll-area {
   flex: 1;