feat: unified header across all pages (Notion-inspired breadcrumb + actions)

Created a shared _header.html template with:
- Hamburger button (toggle sidebar)
- Breadcrumb navigation with clickable segments and separators
- Page icon + title display
- Right-side action buttons (configurable per page)
- Dropdown panel CSS for More/New menus

Updated all pages to use the unified header:
- workspaces.html: Home / Workspaces + login/settings
- workspace.html: Workspace — Projects + login/settings
- local_workspace.html: workspace name
- gitea_workspace.html: Home / owner/repo + New/Upload/Refresh/Settings
- page_editor.html: title + Edited/Share/Copy/Favorite/More dropdown
- settings.html: Settings

Added CSS for breadcrumb, dropdown panel components.

static/css/app.css

@@ -807,6 +807,104 @@ input, select, textarea { font-family: inherit; color: inherit; }
   color: var(--text-secondary);
 }
 
+/* ── Unified Header Breadcrumb ── */
+.header-breadcrumb {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  font-size: 14px;
+  overflow: hidden;
+  flex: 1;
+  min-width: 0;
+}
+
+.header-page-icon {
+  font-size: 16px;
+  flex-shrink: 0;
+  margin-right: 2px;
+}
+
+.breadcrumb-link {
+  color: var(--text-secondary);
+  text-decoration: none;
+  white-space: nowrap;
+  flex-shrink: 0;
+  padding: 2px 4px;
+  border-radius: 4px;
+  transition: background 100ms, color 100ms;
+}
+
+.breadcrumb-link:hover {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+
+.breadcrumb-link.disabled {
+  cursor: default;
+  opacity: 0.6;
+}
+
+.breadcrumb-sep {
+  color: var(--text-tertiary);
+  flex-shrink: 0;
+  font-size: 12px;
+  margin: 0 1px;
+}
+
+.breadcrumb-current {
+  color: var(--text-primary);
+  font-weight: 500;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.header-actions {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  flex-shrink: 0;
+}
+
+/* ── Dropdown panel (reusable) ── */
+.dropdown-panel {
+  position: absolute;
+  display: none;
+  background: var(--bg-secondary);
+  border: 1px solid var(--border-strong);
+  border-radius: 8px;
+  box-shadow: 0 8px 24px rgba(0,0,0,.4);
+  z-index: 1000;
+  padding: 4px;
+  min-width: 160px;
+}
+
+.dropdown-panel.visible {
+  display: block;
+}
+
+.dropdown-option {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 10px;
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 13px;
+  color: var(--text-primary);
+  white-space: nowrap;
+}
+
+.dropdown-option:hover {
+  background: var(--bg-hover);
+}
+
+.option-icon {
+  font-size: 16px;
+  width: 20px;
+  text-align: center;
+}
+
 .topbar-breadcrumb a:hover {
   color: var(--text-primary);
 }