refactor: centralize constants and reorganize documentation

- Create internal/constants package with all hardcoded values
  (environment, cookies, themes, headers, routes, cache)
- Create internal/httputil package for HTTP helper functions
- Update all handlers and middleware to use centralized constants
- Reorganize documentation with numbered prefixes (00-26)
- Remove duplicate docs from validation folder and docs/
- Delete handlers/constants.go (moved to internal/constants)

internal/httputil/method.go

@@ -0,0 +1,25 @@
+package httputil
+
+import (
+	"net/http"
+)
+
+// RequireMethod checks if the request method matches expected.
+// Returns true if method matches, false otherwise (sends 405 error).
+func RequireMethod(w http.ResponseWriter, r *http.Request, method string) bool {
+	if r.Method != method {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return false
+	}
+	return true
+}
+
+// RequirePost is a shorthand for RequireMethod(w, r, http.MethodPost).
+func RequirePost(w http.ResponseWriter, r *http.Request) bool {
+	return RequireMethod(w, r, http.MethodPost)
+}
+
+// RequireGet is a shorthand for RequireMethod(w, r, http.MethodGet).
+func RequireGet(w http.ResponseWriter, r *http.Request) bool {
+	return RequireMethod(w, r, http.MethodGet)
+}