fix(oauth): echo state parameter back in authorization redirect

OAuth 2.0 requires the server to echo the state parameter in the
callback redirect. Mastodon clients (e.g. murmel.social) send a
state value and fail with 'missing parameters' if it is absent.

Thread state through: GET query → session store → hidden form field
→ POST body → callback redirect (approve and deny paths).

lib/mastodon/routes/oauth.js

@@ -206,6 +206,7 @@ router.get("/oauth/authorize", async (req, res, next) => {
       code_challenge,
       code_challenge_method,
       force_login,
+      state,
     } = req.query;
 
     // Restore OAuth params from session after login redirect.
@@ -221,6 +222,7 @@ router.get("/oauth/authorize", async (req, res, next) => {
       scope = p.scope;
       code_challenge = p.code_challenge;
       code_challenge_method = p.code_challenge_method;
+      state = p.state;
     }
 
     if (response_type !== "code") {
@@ -262,7 +264,7 @@ router.get("/oauth/authorize", async (req, res, next) => {
       // login redirect chain due to a re-encoding bug in indieauth.js.
       req.session.pendingOAuth = {
         client_id, redirect_uri, response_type, scope,
-        code_challenge, code_challenge_method,
+        code_challenge, code_challenge_method, state,
       };
       // Redirect to Indiekit's login page with a simple return path.
       return res.redirect("/session/login?redirect=/oauth/authorize");
@@ -300,6 +302,7 @@ router.get("/oauth/authorize", async (req, res, next) => {
     <input type="hidden" name="scope" value="${escapeHtml(requestedScopes.join(" "))}">
     <input type="hidden" name="code_challenge" value="${escapeHtml(code_challenge || "")}">
     <input type="hidden" name="code_challenge_method" value="${escapeHtml(code_challenge_method || "")}">
+    <input type="hidden" name="state" value="${escapeHtml(state || "")}">
     <input type="hidden" name="response_type" value="code">
     <div class="actions">
       <button type="submit" name="decision" value="approve" class="approve">Authorize</button>
@@ -323,6 +326,7 @@ router.post("/oauth/authorize", async (req, res, next) => {
       scope,
       code_challenge,
       code_challenge_method,
+      state,
       decision,
     } = req.body;
 
@@ -365,6 +369,7 @@ router.post("/oauth/authorize", async (req, res, next) => {
           "error_description",
           "The resource owner denied the request",
         );
+        if (state) url.searchParams.set("state", state);
         return redirectToUri(res, redirect_uri, url.toString());
       }
       return res.status(403).json({
@@ -413,9 +418,10 @@ router.post("/oauth/authorize", async (req, res, next) => {
 </html>`);
     }
 
-    // Redirect with code
+    // Redirect with code (and state, which must be echoed back per OAuth 2.0 spec)
     const url = new URL(redirect_uri);
     url.searchParams.set("code", code);
+    if (state) url.searchParams.set("state", state);
     redirectToUri(res, redirect_uri, url.toString());
   } catch (error) {
     next(error);