add tests for controllers and ActionCable channels; improve main backend docs

main-server/spec/models/user_spec.rb

@@ -38,6 +38,27 @@ RSpec.describe User, type: :model do
       expect(duplicate).not_to be_valid
       expect(duplicate.errors[:nickname]).to be_present
     end
+
+    it "accepts valid image formats for avatar" do
+      user = User.new(valid_attributes)
+      user.avatar.attach(
+        io: StringIO.new("\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x01\x00\x00\x00\x01\x08\x06\x00\x00\x00\x1f\x15c4"),
+        filename: "avatar.png",
+        content_type: "image/png"
+      )
+      expect(user).to be_valid
+    end
+
+    it "rejects unsupported mime types for avatar" do
+      user = User.new(valid_attributes)
+      user.avatar.attach(
+        io: StringIO.new("plain text"),
+        filename: "test.txt",
+        content_type: "text/plain"
+      )
+      expect(user).not_to be_valid
+      expect(user.errors[:avatar]).to include("must be a PNG, JPEG, GIF or WebP image")
+    end
   end
 
   describe "rating" do