Refactor: Centralize template context using unified TemplateData struct

- Introduced models.TemplateData for shared user/context state
- Moved context construction logic into handlers/template_context.go
- Simplified helpers.TemplateContext to accept structured data
- Restored and organized template helper functions
- Updated affected handlers (main.go, draw_handler.go, notifications.go)
- Improved scalability and separation of concerns in template rendering

models/template.go

@@ -0,0 +1,10 @@
+package models
+
+type TemplateData struct {
+	User              *User
+	IsAdmin           bool
+	NotificationCount int
+	Notifications     []Notification
+	MessageCount      int
+	Messages          []Message
+}