Sign-in branding: Hesychia.ai + transparent logo (#49) (#50)

## Summary
Closes #49.

- Show **Hesychia.ai** wordmark with the brand mark on the sign-in page
- Regenerate brand PNGs with true transparent corners (source soft-glow RGBA had no fully clear pixels)
- Update SignIn test for the new heading

## Test plan
- [ ] Open `/signin/` — mark + **Hesychia.ai** above Sign In
- [ ] Confirm no black square around mark on dark particle background
- [ ] `npm test -- --testPathPattern=SignIn.test --watchAll=false`Reviewed-on: #50

llm-fe/src/llm-fe/pages/SignIn/SignIn.test.tsx

@@ -57,6 +57,7 @@ describe('SignIn', () => {
   it('renders sign-in form fields', async () => {
     renderSignIn();
 
+    expect(screen.getByRole('heading', { name: 'Hesychia.ai' })).toBeInTheDocument();
     expect(screen.getByRole('heading', { name: 'Sign In' })).toBeInTheDocument();
     expect(screen.getByPlaceholderText('Email Address')).toBeInTheDocument();
     expect(screen.getByPlaceholderText('Password')).toBeInTheDocument();

llm-fe/src/llm-fe/pages/SignIn/SignIn.tsx

@@ -56,13 +56,28 @@ const CardTitle = styled.h2`
   text-align: center;
 `;
 
+const BrandBlock = styled.div`
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  margin-bottom: 1.25rem;
+`;
+
 const BrandMark = styled.img`
   width: 4rem;
   height: 4rem;
-  margin-bottom: 1.25rem;
   border-radius: 50%;
 `;
 
+const BrandWordmark = styled.h1`
+  margin: 0.85rem 0 0;
+  font-size: 1.75rem;
+  font-weight: 700;
+  letter-spacing: 0.02em;
+  color: #fff;
+  text-align: center;
+`;
+
 const StyledInput = styled.input`
   width: 100%;
   background: rgba(255, 255, 255, 0.05);
@@ -243,7 +258,10 @@ const SignIn = (): JSX.Element => {
             <ParticleBackground />
             <ContentWrapper>
                 <GlassCard>
-                    <BrandMark src={hesychiaMark} alt="Hesychia" />
+                    <BrandBlock>
+                        <BrandMark src={hesychiaMark} alt="" />
+                        <BrandWordmark>Hesychia.ai</BrandWordmark>
+                    </BrandBlock>
                     <CardTitle>Sign In</CardTitle>
                     {errorMessage && <ErrorMessage>{errorMessage}</ErrorMessage>}
                     <Formik