feat: normalize email login and improve theme handling

Normalize email input by stripping whitespace and converting to lowercase in login endpoint, remove EmailStr validation in favor of plain string type, move dark mode class toggle from Layout to App component for better initialization, add theme toggle button to login page, and set dark mode as default theme when no preference is stored

backend/app/schemas/domain.py

@@ -1,7 +1,7 @@
 from datetime import datetime
 from typing import Any
 
-from pydantic import BaseModel, ConfigDict, EmailStr, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class OrmModel(BaseModel):
@@ -15,13 +15,13 @@ class TokenPair(BaseModel):
 
 
 class LoginRequest(BaseModel):
-    email: EmailStr
+    email: str
     password: str
 
 
 class UserRead(OrmModel):
     id: str
-    email: EmailStr
+    email: str
     display_name: str
     is_active: bool