Prevent email autofill as display name and autosave profile media

Hop-State: A_06FP8EYP9H40CXQH16XPSN8
Hop-Proposal: R_06FP8EY0KW03D22K71DN4KG
Hop-Task: T_06FP8DNXSH7CAHXCHDZ2Z1R
Hop-Attempt: AT_06FP8DNXSKPDEVNXEZRQ5W8

src/lib/auth/display-name.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from 'vitest';
+import { registrationDisplayName } from './display-name';
+
+describe('registrationDisplayName', () => {
+    it('uses the handle when no display name was provided', () => {
+        expect(registrationDisplayName('alice', 'alice@example.com')).toBe('alice');
+    });
+
+    it('does not expose an autofilled email as the public display name', () => {
+        expect(registrationDisplayName('alice', 'alice@example.com', ' Alice@Example.com ')).toBe('alice');
+    });
+
+    it('trims and preserves an intentional display name', () => {
+        expect(registrationDisplayName('alice', 'alice@example.com', ' Alice Example ')).toBe('Alice Example');
+    });
+});