Add UI button registration API (registerButton, unregisterButton, getButtonsForLocation)

src/PluginInterfaces.ts

@@ -148,6 +148,8 @@ export interface PluginContext {
   ui: {
     registerRenderer: (type: string, renderer: CustomRenderer) => void;
     unregisterRenderer: (type: string) => void;
+    registerButton: (button: UIButtonDefinition) => void;
+    unregisterButton: (buttonId: string) => void;
   };
 
   /** Settings API */
@@ -211,7 +213,55 @@ export type UILocation =
   | 'message-actions'
   | 'user-menu'
   | 'room-menu'
-  | 'composer-actions';
+  | 'composer-actions'
+  | 'channel-list'
+  | 'direct-messages'
+  | 'home-section'
+  | 'search-notification-section'
+  | 'text-composer-toolbar'
+  | 'sidebar-actions';
+
+/**
+ * Button positioning configuration for plugins.
+ * Allows buttons to be placed before or after existing elements or groups.
+ */
+export interface UIButtonPosition {
+  /** Place button before this element/group ID */
+  before?: string;
+  /** Place button after this element/group ID */
+  after?: string;
+  /** 
+   * Optional group ID this button belongs to.
+   * Buttons in the same group are rendered together.
+   */
+  group?: string;
+  /** Order within the group (lower numbers appear first) */
+  order?: number;
+}
+
+/**
+ * Definition for a UI button registered by a plugin.
+ * Framework-agnostic - the component/element is passed as unknown.
+ */
+export interface UIButtonDefinition {
+  /** Unique ID for this button */
+  id: string;
+  /** Location where the button should appear */
+  location: UILocation;
+  /** Button label or tooltip */
+  label: string;
+  /** Optional icon identifier */
+  icon?: string;
+  /** Positioning configuration */
+  position?: UIButtonPosition;
+  /** 
+   * The actual button component or render function.
+   * Framework-agnostic - hosts using React should cast to ReactNode.
+   */
+  component: unknown;
+  /** Optional callback when clicked (if component doesn't handle it) */
+  onClick?: () => void | Promise<void>;
+}
 
 /**
  * The main plugin interface that all plugins must implement.