Fix remaining CSS selector and form field issues from UI/UX overhaul

This addresses the remaining test failures @jasnow reported in issue #486.

Fixes:
1. Ambiguous Login button - Changed from click_button "Login" to
   find("input[type='submit'][value='Login']").click to specifically
   target the form submit button and avoid the header Login button

2. Fixed password_complexity_spec field names:
   - user_email → email
   - user_first_name → first_name
   - user_last_name → last_name
   - user_password → password
   - user_password_confirmation → password_confirmation
   - Submit → Create Account (correct button text)

3. Applied same selector fix to login helper in capybara_shared.rb

These changes complete the test suite fixes for the new UI that was
introduced in the file upload UX improvements.

Related: #486

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

spec/vulnerabilities/password_complexity_spec.rb

@@ -13,14 +13,12 @@ feature "password complexity" do
     new_user_email = normal_user.email + "two"
 
     visit "/signup"
-    within(".signup") do
-      fill_in "user_email", with: new_user_email
-      fill_in "user_first_name", with: normal_user.first_name
-      fill_in "user_last_name", with: normal_user.last_name + "not"
-      fill_in "user_password", with: "password"
-      fill_in "user_password_confirmation", with: "password"
-    end
-    click_on "Submit"
+    fill_in "email", with: new_user_email
+    fill_in "first_name", with: normal_user.first_name
+    fill_in "last_name", with: normal_user.last_name + "not"
+    fill_in "password", with: "password"
+    fill_in "password_confirmation", with: "password"
+    click_on "Create Account"
 
     expect(User.find_by(email: new_user_email)).to be_nil
     expect(current_path).to eq("/signup")