refactor: Extract shared utilities and add validation layer

Part 1: Shared Utilities
- Create internal/fileutil package with FindDataFile() and LoadJSON()
- Create internal/lang package with language constants and validation
- Eliminate 46 lines of code duplication between cv/loader.go and ui/loader.go
- Simplify cv/loader.go from 69 to 36 lines (-48%)
- Simplify ui/loader.go from 56 to 24 lines (-57%)

Part 2: Validation Layer
- Add comprehensive validation in internal/models/cv/validation.go
- Validate Personal (name, email format, URLs)
- Validate Experience (required fields, dates)
- Validate Education (required fields)
- Validate Skills (proficiency ranges 1-5, categories)
- Validate Languages (proficiency levels 1-5)
- Validate Projects (title, URLs)
- Validate Meta (version, language)
- Integrate validation into LoadCV() - automatic on load
- Create ValidationError and ValidationErrors types for clear error reporting
- Report all validation errors at once (better UX)

Testing:
- Add comprehensive tests for fileutil package (FindDataFile, LoadJSON)
- Add tests for lang package (IsValid, Validate, All)
- Add 280+ validation test cases covering edge cases
- All tests pass with real CV data (cv-en.json, cv-es.json)
- Fixed validation to allow both URLs and local paths for gitRepoUrl

Documentation:
- Create _go-learning/refactorings/002-shared-utilities-validation.md
- Document architecture, benefits, testing, and interview talking points
- Explain WHY decisions were made (DRY, type safety, data integrity)

Benefits:
- DRY: Single source of truth for utilities
- Type safety: Language constants instead of magic strings
- Data integrity: Validation catches errors at load time
- Better errors: Clear messages showing all issues at once
- Maintainability: Centralized utilities easier to update

internal/lang/lang_test.go

@@ -0,0 +1,121 @@
+package lang_test
+
+import (
+	"testing"
+
+	"github.com/juanatsap/cv-site/internal/lang"
+)
+
+func TestIsValid(t *testing.T) {
+	tests := []struct {
+		name     string
+		language string
+		want     bool
+	}{
+		{
+			name:     "Valid - English",
+			language: "en",
+			want:     true,
+		},
+		{
+			name:     "Valid - Spanish",
+			language: "es",
+			want:     true,
+		},
+		{
+			name:     "Invalid - French",
+			language: "fr",
+			want:     false,
+		},
+		{
+			name:     "Invalid - Empty",
+			language: "",
+			want:     false,
+		},
+		{
+			name:     "Invalid - Uppercase",
+			language: "EN",
+			want:     false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := lang.IsValid(tt.language); got != tt.want {
+				t.Errorf("IsValid() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestValidate(t *testing.T) {
+	tests := []struct {
+		name     string
+		language string
+		wantErr  bool
+	}{
+		{
+			name:     "Valid - English",
+			language: "en",
+			wantErr:  false,
+		},
+		{
+			name:     "Valid - Spanish",
+			language: "es",
+			wantErr:  false,
+		},
+		{
+			name:     "Invalid - French",
+			language: "fr",
+			wantErr:  true,
+		},
+		{
+			name:     "Invalid - Empty",
+			language: "",
+			wantErr:  true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := lang.Validate(tt.language)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Validate() error = %v, wantErr %v", err, tt.wantErr)
+			}
+			if err != nil && tt.wantErr {
+				// Check that error message includes supported languages
+				errMsg := err.Error()
+				if errMsg == "" {
+					t.Error("Validate() error message is empty")
+				}
+			}
+		})
+	}
+}
+
+func TestAll(t *testing.T) {
+	all := lang.All()
+
+	if len(all) != 2 {
+		t.Errorf("All() returned %d languages, want 2", len(all))
+	}
+
+	// Check that it contains en and es
+	hasEN := false
+	hasES := false
+	for _, l := range all {
+		if l == "en" {
+			hasEN = true
+		}
+		if l == "es" {
+			hasES = true
+		}
+	}
+
+	if !hasEN {
+		t.Error("All() missing 'en'")
+	}
+	if !hasES {
+		t.Error("All() missing 'es'")
+	}
+}