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/cv_cmdk.go

@@ -4,6 +4,9 @@ import (
 	"encoding/json"
 	"log"
 	"net/http"
+
+	"github.com/juanatsap/cv-site/internal/constants"
+	"github.com/juanatsap/cv-site/internal/httputil"
 )
 
 // CmdKAction represents a single action for the ninja-keys command palette
@@ -25,14 +28,7 @@ type CmdKResponse struct {
 // This endpoint provides dynamic entries for experiences, projects, and courses
 // that can be searched via CMD+K
 func (h *CVHandler) CmdKData(w http.ResponseWriter, r *http.Request) {
-	// Get language from query parameter, default to "en"
-	lang := r.URL.Query().Get("lang")
-	if lang == "" {
-		lang = "en"
-	}
-	if lang != "en" && lang != "es" {
-		lang = "en"
-	}
+	lang := httputil.Lang(r)
 
 	// Get CV data from cache
 	cv := h.dataCache.GetCV(lang)
@@ -93,8 +89,8 @@ func (h *CVHandler) CmdKData(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Set headers and encode response
-	w.Header().Set("Content-Type", "application/json")
-	w.Header().Set("Cache-Control", "public, max-age=3600") // Cache for 1 hour
+	w.Header().Set(constants.HeaderContentType, constants.ContentTypeJSON)
+	w.Header().Set(constants.HeaderCacheControl, constants.CachePublic1Hour)
 
 	if err := json.NewEncoder(w).Encode(response); err != nil {
 		log.Printf("ERROR encoding CMD+K response: %v", err)