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/store/pluginToolbarStore.ts

@@ -0,0 +1,49 @@
+import { create } from 'zustand';
+
+export interface ToolbarItem {
+  id: string;
+  plugin: string;
+  label: string;
+  icon?: React.ReactNode;
+  onClick: () => void;
+  group?: string;
+  disabled?: boolean;
+  active?: boolean;
+  type?: 'button' | 'search' | 'select';
+  searchPlaceholder?: string;
+  onSearch?: (query: string) => void;
+  selectOptions?: { value: string; label: string }[];
+  selectValue?: string;
+  onSelect?: (value: string) => void;
+}
+
+interface PluginToolbarState {
+  items: ToolbarItem[];
+  activePlugin: string | null;
+  registerItems: (plugin: string, items: ToolbarItem[]) => void;
+  unregisterPlugin: (plugin: string) => void;
+  setActivePlugin: (plugin: string | null) => void;
+  updateItem: (plugin: string, id: string, updates: Partial<ToolbarItem>) => void;
+}
+
+export const usePluginToolbarStore = create<PluginToolbarState>((set) => ({
+  items: [],
+  activePlugin: null,
+  registerItems: (plugin, newItems) =>
+    set((state) => ({
+      items: [...state.items.filter((i) => i.plugin !== plugin), ...newItems],
+      activePlugin: plugin,
+    })),
+  unregisterPlugin: (plugin) =>
+    set((state) => ({
+      items: state.items.filter((i) => i.plugin !== plugin),
+      activePlugin: state.activePlugin === plugin ? null : state.activePlugin,
+    })),
+  setActivePlugin: (plugin) => set({ activePlugin: plugin }),
+  updateItem: (plugin, id, updates) =>
+    set((state) => ({
+      items: state.items.map((i) =>
+        i.plugin === plugin && i.id === id ? { ...i, ...updates } : i
+      ),
+    })),
+}));