Adapt OAuth2 implementation to Matrix Spec v1.18 (#34026)

* Adapt OAuth2 implementation to Matrix Spec v1.18

* Handle more cases of oidc->oauth

* Fix test

* Fix read back of oauth2 context

* Iterate

* Fix tests

* Discard changes to apps/web/playwright/e2e/settings/account-user-settings-tab.spec.ts

* Fix test

* Fix test

* Fix test

* Potential fix for pull request finding 'Unused variable, import, function or class'

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

* Iterate

* Iterate

* Fix test

---------

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

docs/.vitepress/config.ts

@@ -137,7 +137,7 @@ export default withMermaid({
                     { text: "Memory profiling", link: "/memory-profiles-and-leaks.md" },
                     { text: "Jitsi", link: "/jitsi-dev.md" },
                     { text: "Feature flags", link: "/feature-flags.md" },
-                    { text: "OIDC and delegated authentication", link: "/oidc.md" },
+                    { text: "OAuth and delegated authentication", link: "/oauth.md" },
                     { text: "Release Process", link: "/release.md" },
                     { text: "MVVM", link: "/MVVM.md" },
                     { text: "Settings", link: "/settings.md" },

docs/config.md

@@ -298,7 +298,6 @@ The following subproperties are available:
 2. `logo_uri`: Optional URI for the client logo.
 3. `tos_uri`: Optional URI for the client's terms of service.
 4. `policy_uri`: Optional URI for the client's privacy policy.
-5. `contacts`: Optional list of contact emails for the client.
 
 As an example:
 
@@ -308,8 +307,7 @@ As an example:
         "client_uri": "https://example.com",
         "logo_uri": "https://example.com/logo.png",
         "tos_uri": "https://example.com/tos",
-        "policy_uri": "https://example.com/policy",
-        "contacts": ["support@example.com"]
+        "policy_uri": "https://example.com/policy"
     }
 }
 ```

docs/oauth.md

@@ -1,13 +1,13 @@
-# OIDC and delegated authentication
+# OAuth2 and delegated authentication
 
 See https://areweoidcyet.com/client-implementation-guide/ for implementation details.
 
-Element Web uses [MSC2965: OIDC provider discovery](https://github.com/matrix-org/matrix-spec-proposals/pull/2965) to discover the configured provider.
-Where a valid MSC2965 configuration is discovered, OIDC native login flow will be the only login option offered.
-Element Web will attempt to [dynamically register](https://openid.net/specs/openid-connect-registration-1_0.html) with the configured OP.
+Element Web uses [/auth_metadata](https://spec.matrix.org/v1.18/client-server-api/#get_matrixclientv1auth_metadata) to discover the configured provider.
+Where a valid configuration is discovered, OAuth2 native login flow will be the only login option offered.
+Element Web will attempt to [dynamically register](https://spec.matrix.org/v1.18/client-server-api/#client-registration) with the configured OP.
 Then, authentication will be completed [as described here](https://areweoidcyet.com/client-implementation-guide/).
 
-#### Statically configured OIDC clients
+#### Statically configured OAuth2 clients
 
 Clients that are already registered with the OP can configure their `client_id` in `config.json`.
 Where static configuration exists for the OP dynamic client registration will not be attempted.