Fix Docker signup, refresh docs, and improve blank invoice flow.

Parse DISABLE_SIGNUPS and related env booleans correctly for Compose string values, and derive auth trustedOrigins from BETTER_AUTH_URL. Rewrite README and architecture docs with the git.soconnor.dev remote and accurate deployment guidance. Allow zero-line-item draft invoices with validation when sending email.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/server/api/routers/email.ts

@@ -70,6 +70,10 @@ export const emailRouter = createTRPCRouter({
         throw new Error("Client has no email address");
       }
 
+      if (!invoice.items.length) {
+        throw new Error("Add at least one line item before sending this invoice");
+      }
+
       // Validate email format
       const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
       if (!emailRegex.test(invoice.client.email)) {