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/handlers/errors.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"log"
 	"net/http"
+
+	"github.com/juanatsap/cv-site/internal/constants"
 )
 
 // ErrorResponse represents a structured error response
@@ -62,12 +64,12 @@ func HandleError(w http.ResponseWriter, r *http.Request, err error) {
 
 	// Determine response based on Accept header
 	accept := r.Header.Get("Accept")
-	isJSON := accept == "application/json"
-	isHTMX := r.Header.Get("HX-Request") != ""
+	isJSON := accept == constants.ContentTypeJSON
+	isHTMX := r.Header.Get(constants.HeaderHXRequest) != ""
 
 	if isJSON {
 		// JSON response
-		w.Header().Set("Content-Type", "application/json")
+		w.Header().Set(constants.HeaderContentType, constants.ContentTypeJSON)
 		w.WriteHeader(appErr.StatusCode)
 
 		response := ErrorResponse{
@@ -88,7 +90,7 @@ func HandleError(w http.ResponseWriter, r *http.Request, err error) {
 
 	if isHTMX {
 		// HTMX response - return simple HTML fragment
-		w.Header().Set("Content-Type", "text/html")
+		w.Header().Set(constants.HeaderContentType, constants.ContentTypeHTMLFragment)
 		w.WriteHeader(appErr.StatusCode)
 
 		message := appErr.Message