feat: plugin ribbon toolbar + mail response fixes

- Add pluginToolbarStore for plugin toolbar item registration
- Add PluginToolbar component (slim ribbon bar under TopBar)
- Update AppShell to render PluginToolbar between TopBar and content
- Mail page registers toolbar items (account select, search, compose, sync, settings)
- Remove old mail top bar, use plugin toolbar instead
- Fix mail response: arrays for addresses, add from_name/body_html/sanitized_html/date

frontend/src/components/layout/AppShell.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import { Outlet } from 'react-router-dom';
 import { Sidebar } from './Sidebar';
 import { TopBar } from './TopBar';
+import { PluginToolbar } from './PluginToolbar';
 import { ToastContainer } from '@/components/ui/Toast';
 
 export function AppShell() {
@@ -10,6 +11,7 @@ export function AppShell() {
       <Sidebar />
       <div className="flex-1 flex flex-col overflow-hidden">
         <TopBar />
+        <PluginToolbar />
         <main
           className="flex-1 overflow-y-auto focus:outline-none"
           tabIndex={-1}