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/gitea_workspace.html

@@ -4,22 +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">
-    <a href="/workspaces" class="breadcrumb" style="color:rgba(255,255,255,.5);text-decoration:none;">Home</a>
-    <span class="page-title-topbar">{{ owner }}/{{ repo }}</span>
-  </div>
-  <div class="topbar-right" style="display:flex;align-items:center;gap:6px;">
-    <button class="page-action-btn" @click="showNewFile=!showNewFile" title="New file">📄+</button>
-    <button class="page-action-btn" @click="triggerUpload()" title="Upload file">📤</button>
-    <input type="file" id="gitea-upload-input" style="display:none" @change="doUpload($event)" multiple>
-    <button class="page-action-btn" @click="refreshTree()" title="Refresh">🔄</button>
-    <button class="page-action-btn" onclick="window.location='/accounts/settings'" title="Settings">⚙</button>
-  </div>
-</header>
+{% set breadcrumb_items = [{"label": "Home", "url": "/workspaces"}, {"label": owner ~ "/" ~ repo, "url": None}] %}
+{% set page_icon = "🔗" %}
+{% set right_actions = '<button class="topbar-btn" @click="showNewFile=!showNewFile" title="New file">📄+</button><button class="topbar-btn" @click="triggerUpload()" title="Upload file">📤</button><input type="file" id="gitea-upload-input" style="display:none" @change="doUpload($event)" multiple><button class="topbar-btn" @click="refreshTree()" title="Refresh">🔄</button><a href="/accounts/settings" class="topbar-btn" title="Settings">⚙</a>' %}
+{% include '_header.html' %}
 {% endblock %}
 
 {% block content %}