fix: preserve shared screenshots through sign-in (Closes #741)

frontend/login.js

@@ -4,7 +4,7 @@
 }(typeof self !== 'undefined' ? self : this, function createLoginController(options) {
   function validShareContinuation(value) {
     if (typeof value !== 'string' || !value.startsWith('./?') || value.includes('#')) return './';
-    const limits = { title: 200, text: 8000, url: 2048 };
+    const limits = { title: 200, text: 8000, url: 2048, launch: 8, shared: 5 };
     const params = new URLSearchParams(value.slice(3));
     const entries = Array.from(params.entries());
     if (!entries.length) return './';
@@ -14,6 +14,11 @@
       || content.length > limits[name]
     ));
     if (invalid) return './';
+    if (Object.keys(limits).some(name => params.getAll(name).length > 1)) return './';
+    const launch = params.get('launch');
+    const shared = params.get('shared');
+    if (launch && !['continue', 'new', 'agenda'].includes(launch)) return './';
+    if (shared !== null && (shared !== 'image' || launch !== 'new')) return './';
     return value;
   }