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`

test/unit-tests/vector/platform/ElectronPlatform-test.ts

@@ -132,7 +132,7 @@ describe("ElectronPlatform", () => {
         new ElectronPlatform();
         dispatcher.dispatch(
             {
-                action: "client_started",
+                action: Action.ClientStarted,
             },
             true,
         );

test/unit-tests/vector/platform/WebPlatform-test.ts

@@ -15,6 +15,7 @@ import { setupLanguageMock } from "../../../setup/setupLanguage";
 import ToastStore from "../../../../src/stores/ToastStore.ts";
 import defaultDispatcher from "../../../../src/dispatcher/dispatcher.ts";
 import { emitPromise } from "../../../test-utils";
+import { Action } from "../../../../src/dispatcher/actions.ts";
 
 fetchMock.config.overwriteRoutes = true;
 
@@ -49,7 +50,7 @@ describe("WebPlatform", () => {
             };
             new WebPlatform();
 
-            defaultDispatcher.dispatch({ action: "client_started" });
+            defaultDispatcher.dispatch({ action: Action.ClientStarted });
             await emitPromise(ToastStore.sharedInstance(), "update");
             const toasts = ToastStore.sharedInstance().getToasts();
             expect(toasts).toHaveLength(1);