feat: add Notion-style breadcrumb navigation with dropdown menus

Backend:
- Added _nav_breadcrumb() to build page hierarchy chain (root → current)
- New /api/nav/menu endpoint returns workspace/folder children with has_children flag
- view_page() and view_page_root() now pass breadcrumb_items, nav_workspace_id, nav_page_id to template

Header template (_header.html):
- Replaced static breadcrumb with Alpine.js fdBreadcrumb() component
- Breadcrumb items now clickable with dropdown menus showing

static/css/app.css

@@ -843,13 +843,25 @@ input, select, textarea { font-family: inherit; color: inherit; }
 .header-breadcrumb {
   display: flex;
   align-items: center;
-  gap: 4px;
+  gap: 2px;
   font-size: 14px;
-  overflow: hidden;
+  overflow: visible;
   flex: 1;
   min-width: 0;
 }
 
+.crumb-cell {
+  display: inline-flex;
+  align-items: center;
+  min-width: 0;
+}
+
+.crumb-anchor {
+  position: relative;
+  display: inline-flex;
+  align-items: center;
+}
+
 .header-page-icon {
   font-size: 16px;
   flex-shrink: 0;
@@ -861,7 +873,16 @@ input, select, textarea { font-family: inherit; color: inherit; }
   text-decoration: none;
   white-space: nowrap;
   flex-shrink: 0;
-  padding: 2px 4px;
+  max-width: 220px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  padding: 2px 6px;
+  border: none;
+  background: transparent;
+  font: inherit;
+  font-size: 14px;
+  line-height: 1.4;
+  cursor: pointer;
   border-radius: 4px;
   transition: background 100ms, color 100ms;
 }
@@ -876,11 +897,17 @@ input, select, textarea { font-family: inherit; color: inherit; }
   opacity: 0.6;
 }
 
+.crumb-ellipsis {
+  font-weight: 600;
+  letter-spacing: 1px;
+}
+
 .breadcrumb-sep {
   color: var(--text-tertiary);
   flex-shrink: 0;
-  font-size: 12px;
+  font-size: 13px;
   margin: 0 1px;
+  user-select: none;
 }
 
 .breadcrumb-current {
@@ -891,6 +918,8 @@ input, select, textarea { font-family: inherit; color: inherit; }
   text-overflow: ellipsis;
 }
 
+button.breadcrumb-current { max-width: 260px; }
+
 .header-actions {
   display: flex;
   align-items: center;
@@ -898,6 +927,80 @@ input, select, textarea { font-family: inherit; color: inherit; }
   flex-shrink: 0;
 }
 
+/* ── Breadcrumb navigation dropdown (Notion-style) ── */
+.fd-nav-menu {
+  position: absolute;
+  top: calc(100% + 4px);
+  left: 0;
+  min-width: 240px;
+  max-width: 320px;
+  overflow: visible;
+  background: var(--bg-modal, #fff);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-lg, 8px);
+  box-shadow: var(--shadow-popover, 0 6px 24px rgba(0,0,0,0.25));
+  z-index: 2000;
+  padding: 6px;
+}
+
+.fd-nav-submenu {
+  top: -6px;
+  left: calc(100% + 2px);
+}
+
+.fd-nav-section {
+  padding: 4px 8px 6px;
+  font-size: 11px;
+  font-weight: 600;
+  letter-spacing: 0.02em;
+  color: var(--text-tertiary);
+  text-transform: none;
+}
+
+.fd-nav-empty {
+  padding: 8px 10px;
+  font-size: 13px;
+  color: var(--text-tertiary);
+}
+
+.fd-nav-item {
+  position: relative;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 8px;
+  border-radius: 6px;
+  font-size: 14px;
+  color: var(--text-primary);
+  cursor: pointer;
+  white-space: nowrap;
+  transition: background 100ms;
+}
+
+.fd-nav-item:hover {
+  background: var(--bg-hover);
+}
+
+.fd-nav-ico {
+  font-size: 16px;
+  width: 20px;
+  text-align: center;
+  flex-shrink: 0;
+}
+
+.fd-nav-label {
+  flex: 1;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.fd-nav-arrow {
+  color: var(--text-tertiary);
+  font-size: 16px;
+  flex-shrink: 0;
+  margin-left: 4px;
+}
+
 /* ── Dropdown panel (reusable) ── */
 .dropdown-panel {
   position: absolute;