Add image upload preview functionality; update titles and templates for consistency

Co-authored-by: Copilot <copilot@github.com>

frontend/tests/test_frontend.py

@@ -196,6 +196,7 @@ def test_generate_image_page_renders(client):
     resp = client.get("/generate/image")
     assert resp.status_code == 200
     assert b"Image Generation" in resp.data
+    assert b"reference_image" in resp.data
 
 
 def test_generate_image_success(client):