Documentation and symbolic constants for dispatcher actions (#31278)

* Remove unreachable code

`view_last_screen` is never used.

* Remove unused action `user_activity_started`

Nothing listens to this, so it's pointless.

* Symbolic constant for `Action.UserActivity`

* Define symbolic constants for more `Action`s

Constants for some actions that are emitted by `Lifecycle`

src/components/structures/EmbeddedPage.tsx

@@ -18,6 +18,7 @@ import { MatrixClientPeg } from "../../MatrixClientPeg";
 import MatrixClientContext from "../../contexts/MatrixClientContext";
 import AutoHideScrollbar from "./AutoHideScrollbar";
 import { type ActionPayload } from "../../dispatcher/payloads";
+import { Action } from "../../dispatcher/actions.ts";
 
 interface IProps {
     // URL to request embedded page content from
@@ -109,7 +110,7 @@ export default class EmbeddedPage extends React.PureComponent<IProps, IState> {
 
     private onAction = (payload: ActionPayload): void => {
         // HACK: Workaround for the context's MatrixClient not being set up at render time.
-        if (payload.action === "client_started") {
+        if (payload.action === Action.ClientStarted) {
             this.forceUpdate();
         }
     };

src/components/structures/MatrixChat.tsx

@@ -812,13 +812,6 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
                 }
                 break;
             }
-            case "view_last_screen":
-                // This function does what we want, despite the name. The idea is that it shows
-                // the last room we were looking at or some reasonable default/guess. We don't
-                // have to worry about email invites or similar being re-triggered because the
-                // function will have cleared that state and not execute that path.
-                this.showScreenAfterLogin();
-                break;
             case "hide_left_panel":
                 this.setState(
                     {
@@ -856,13 +849,13 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
                     this.onLoggedIn();
                 }
                 break;
-            case "on_client_not_viable":
+            case Action.ClientNotViable:
                 this.onSoftLogout();
                 break;
             case Action.OnLoggedOut:
                 this.onLoggedOut();
                 break;
-            case "will_start_client":
+            case Action.WillStartClient:
                 this.setState({ ready: false }, () => {
                     // if the client is about to start, we are, by definition, not ready.
                     // Set ready to false now, then it'll be set to true when the sync
@@ -870,7 +863,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
                     this.onWillStartClient();
                 });
                 break;
-            case "client_started":
+            case Action.ClientStarted:
                 // No need to make this handler async to wait for the result of this
                 this.onClientStarted().catch((e) => {
                     logger.error("Exception in onClientStarted", e);