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.

app/templates/base.html

@@ -329,47 +329,7 @@
 
     <!-- Topbar -->
     {% block topbar %}
-    <header class="topbar">
-      <!-- Hamburger / uncollapse sidebar -->
-      <button class="hamburger-btn" @click="sidebarCollapsed ? (sidebarCollapsed = false) : (mobileSidebarOpen = true, sidebarCollapsed = false)" title="Menu">
-        <svg width="22" height="22" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><line x1="3" y1="6" x2="21" y2="6"/><line x1="3" y1="12" x2="21" y2="12"/><line x1="3" y1="18" x2="21" y2="18"/></svg>
-      </button>
-      <div class="topbar-left">
-        <div class="topbar-breadcrumb">
-          <span x-text="workspaceName"></span>
-          <span class="text-dim">/</span>
-          <span class="text-dim" x-text="currentPage"></span>
-        </div>
-      </div>
-
-      <div class="topbar-center">
-        <div class="topbar-page-title">
-          <span class="page-icon" x-text="pageIcon"></span>
-          <span x-text="pageTitle"></span>
-        </div>
-        <div class="topbar-page-status">
-          <span>🔒</span> Private
-          <span style="cursor:pointer;">▾</span>
-        </div>
-      </div>
-
-      <div class="topbar-right">
-        <span class="text-xs text-dim">Edited <span x-text="lastEdited"></span></span>
-        <button class="topbar-btn">🔔</button>
-        <button class="topbar-btn">🕒</button>
-        <button class="topbar-btn">🔍</button>
-        <button class="topbar-btn share-btn">🔗 Share ▾</button>
-        <button class="topbar-btn">⭐</button>
-        <button class="topbar-btn" @click="showNewPageMenu = !showNewPageMenu" style="position:relative;">
-          New ▾
-          <div class="dropdown-panel" :class="{ visible: showNewPageMenu }" style="top:100%; right:0; left:auto; min-width:200px;" @click.stop>
-            <div class="dropdown-option" @click="createEmptyPage()"><span class="option-icon">📄</span> Empty page</div>
-            <div class="dropdown-option" @click="createDatabase()"><span class="option-icon">📊</span> Database</div>
-            <div class="dropdown-option" @click="createBoard()"><span class="option-icon">📋</span> Kanban board</div>
-          </div>
-        </button>
-      </div>
-    </header>
+    {% include '_header.html' %}
     {% endblock %}
 
     <!-- Content -->