refactor(oauth): drop the hand-written providers and keep only what the SDK ships

The previous commit added five direct clients (Douyin, Microsoft, Facebook,
Twitter, Apple) written against each platform's own endpoints. Maintaining
five bespoke OAuth implementations — including Apple's ES256 client secret and
Twitter's PKCE — is maintenance the bundled yurunsoft SDK does not ask for,
and none of them could be verified without production credentials.

Only providers yurunsoft/yurun-oauth-login actually implements stay on the
direct surface: GitHub, Gitee, QQ, Google, Weibo, Weixin, Baidu and Alipay.
Baidu and Alipay move into OAuthProviderBroker's existing client() switch and
normalize() map rather than living in a separate class, so there is no new
layer to reason about.

OAuthProviderClient, its harness and its contract test are deleted outright
(-809 lines). The Apple form_post handling added to handle_callback() is
reverted too — without Apple there is no POST callback to read.

The aggregate-login channel keeps its 16 platforms, matching the reference;
that list describes what the aggregation provider offers, not a local
implementation. Brand icons for those names stay for the same reason.

Verification: 869 widget/navigation tests pass, the PSR-4 audit reports 0
errors, and the catalog constant, PHP schema and Vue schema now list the same
16 aggregate platforms.

src/Auth/OAuthProviderCatalog.php

@@ -42,11 +42,6 @@ final class OAuthProviderCatalog {
 			'id_key' => 'weixin_appid',
 			'secret_key' => 'weixin_appsecret',
 		],
-		'douyin' => [
-			'label' => '抖音',
-			'id_key' => 'douyin_appid',
-			'secret_key' => 'douyin_appkey',
-		],
 		'baidu' => [
 			'label' => '百度',
 			'id_key' => 'baidu_appid',
@@ -58,31 +53,6 @@ final class OAuthProviderCatalog {
 			'secret_key' => 'alipay_appkey',
 			'extra_keys' => ['private_key' => 'alipay_private_key'],
 		],
-		'microsoft' => [
-			'label' => 'Microsoft',
-			'id_key' => 'microsoft_appid',
-			'secret_key' => 'microsoft_appkey',
-		],
-		'facebook' => [
-			'label' => 'Facebook',
-			'id_key' => 'facebook_appid',
-			'secret_key' => 'facebook_appkey',
-		],
-		'twitter' => [
-			'label' => 'Twitter',
-			'id_key' => 'twitter_appid',
-			'secret_key' => 'twitter_appkey',
-		],
-		'apple' => [
-			'label' => 'Apple',
-			'id_key' => 'apple_appid',
-			'secret_key' => 'apple_appkey',
-			'extra_keys' => [
-				'team_id' => 'apple_team_id',
-				'key_id' => 'apple_key_id',
-				'private_key' => 'apple_private_key',
-			],
-		],
 	];
 
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