chore: Pre-register a builtin OAuth2 application for the official Gitea mobile app (#38880)

This is a prepare and required step for upcoming Gitea Official Mobile
APP which supports login with OAuth2.

The official Gitea mobile app needs the same mechanism. This adds a
builtin application for it:

| | |
|---|---|
| client ID | `b757811a-05c8-4c76-8d74-a5ee3d2073f2` |
| config name | `gitea-app` |
| display name | `Gitea App` |
| redirect URI | `com.gitea.app://oauth/callback` |

Unlike the existing entries, which are CLIs and can therefore use a
loopback `http://127.0.0.1` redirect, a mobile app authorises through a
system browser session (`ASWebAuthenticationSession` on iOS, Custom Tabs
on Android) that can only receive a custom-scheme callback, hence the
custom scheme here.

models/auth/oauth2.go

@@ -83,6 +83,11 @@ func BuiltinApplications() map[string]*BuiltinOAuth2Application {
 		DisplayName:  "tea",
 		RedirectURIs: []string{"http://127.0.0.1", "https://127.0.0.1"},
 	}
+	m["b757811a-05c8-4c76-8d74-a5ee3d2073f2"] = &BuiltinOAuth2Application{
+		ConfigName:   "gitea-app",
+		DisplayName:  "Gitea App",
+		RedirectURIs: []string{"com.gitea.app://oauth/callback"},
+	}
 	return m
 }