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/workspaces.html

@@ -4,18 +4,10 @@
 {% block page_icon %}🏠{% endblock %}
 
 {% block topbar %}
-<header class="topbar">
-  <button class="hamburger-btn" @click="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">
-    <span class="breadcrumb" style="color:rgba(255,255,255,.5)">Home</span>
-    <span class="page-title-topbar">Workspaces</span>
-  </div>
-  <div class="topbar-right">
-    <button class="page-action-btn" onclick="window.location='/accounts/settings'" title="Settings">⚙</button>
-  </div>
-</header>
+{% set breadcrumb_items = [{"label": "Home", "url": "/workspaces"}, {"label": "Workspaces", "url": None}] %}
+{% set page_icon = "🏠" %}
+{% set right_actions = '<a href="/auth/login?provider=local" class="topbar-btn" title="Login">🔑</a><a href="/accounts/settings" class="topbar-btn" title="Settings">⚙</a>' %}
+{% include '_header.html' %}
 {% endblock %}
 
 {% block content %}