Complete "Entering / exiting fullscreen" pattern documentation

Add `App_onhostcontextchanged_respondToDisplayMode` example showing how to
listen for display mode changes and update UI with `classList.toggle()`.

Update `App_requestDisplayMode_fullscreen` to `App_requestDisplayMode_toggle`
to show the common toggle pattern checking `availableDisplayModes` first.

Refactor full examples (pdf-server, shadertoy-server) to use
`classList.toggle(class, condition)` instead of add/remove branches.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app.ts

@@ -729,8 +729,8 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
    * @param options - Request options (timeout, etc.)
    * @returns Result containing the actual display mode that was set
    *
-   * @example Request fullscreen mode
-   * {@includeCode ./app.examples.ts#App_requestDisplayMode_fullscreen}
+   * @example Toggle display mode
+   * {@includeCode ./app.examples.ts#App_requestDisplayMode_toggle}
    *
    * @see {@link McpUiRequestDisplayModeRequest} for request structure
    * @see {@link McpUiHostContext} for checking availableDisplayModes