chore(startup): name the WebView2 runtime and mark every milestone

His startup.log stops after 'data dir ok', which rules out the two faults
already covered and leaves the window creation itself — where a missing
WebView2 runtime lands, sometimes as a silent exit rather than an error
anyone can print.

So the runtime's version is read from where it registers itself (both
hives, and the 32-bit view where a per-machine install usually lands) and
written BEFORE the window is attempted: 'WebView2: not found' answers the
whole question in one line. Breadcrumbs now also mark entering wails.Run,
reaching OnStartup, and the window painting — which places any launch
that produces nothing between two known points.

webview2_other.go

@@ -0,0 +1,7 @@
+//go:build !windows
+
+package main
+
+// webView2Version is a Windows question; elsewhere the browser engine comes
+// with the platform.
+func webView2Version() string { return "n/a" }