Make Glaze native-GUI only and fail fast on missing WebView

[turinglambdaai]

Summary

Make Glaze consistently behave as a native desktop GUI framework instead of silently degrading into a browser application.

GUI-first behavior

• raco glaze init now scaffolds a native run-app entry point.
• raco glaze dev runs the project's actual main.rkt, so development and packaged apps use the same native path.
• Removed #:fallback-browser? from run-app, open-window, and open-webview.
• Native WebView startup failures are fatal; the underlying backend error is preserved.
• If run-app has already started its local server and native window creation fails, the server is shut down before the exception is propagated.
• open-browser remains only as an explicit utility for external documentation/OAuth/support URLs; it is never an application-startup fallback.

Actionable dependency feedback

• Windows: directs users to Microsoft Edge WebView2 Runtime (Evergreen), includes the winget command and Microsoft's official download page. Glaze still ships WebView2Loader.dll.
• Linux: names GTK 3 / WebKitGTK packages and the graphical-session requirement.
• macOS: explains that WKWebView is built into macOS and preserves the initialization error for diagnosis.
• Packaged GUI applications also attempt to show an OS-level error dialog before exiting, because Windows raco exe --gui applications may have no visible console. CI suppresses dialogs automatically; GLAZE_NO_STARTUP_DIALOG=1 disables them explicitly.

CLI, packaging, examples, and docs

• The generated main.rkt uses top-level run-app; this is required because build-app's generated wrapper loads the entry with require, so a module+ main-only scaffold would not start after packaging.
• examples/webview-demo.rkt is now explicitly cross-platform and native-only.
• README / README.zh-CN / Scribble / maintainer docs now describe the GUI-first contract and platform prerequisites consistently.
• Added docs/gui-first.md.

Tests and fixes exposed by CI

• Added glaze-test/gui-first-test.rkt to prevent reintroduction of #:fallback-browser? and verify dependency guidance.
• Fixed the AppKit menu e2e gate so the macOS-only Objective-C test cannot run on Windows merely because WebView2 is available.
• Fixed two small pre-existing CLI parser issues encountered while touching the scaffold: --timestamp-url now preserves URL schemes, and license sign|verify subcommand detection correctly handles the initial #f state.

Compatibility note

This intentionally removes the old browser-fallback API. Callers that passed #:fallback-browser? must remove it; native WebView availability is now a startup prerequisite.