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/fileutil/fileutil.go

@@ -0,0 +1,48 @@
+package fileutil
+
+import (
+	"fmt"
+	"os"
+)
+
+// FindDataFile locates a data file by searching up the directory tree.
+// This is useful for tests that may run from different directory depths.
+//
+// It searches in the following order:
+//  1. Current directory
+//  2. One level up (../)
+//  3. Two levels up (../../)
+//  4. Three levels up (../../../)
+//
+// Example:
+//
+//	path, err := fileutil.FindDataFile("data/cv-en.json")
+//	if err != nil {
+//	    log.Fatal(err)
+//	}
+func FindDataFile(filename string) (string, error) {
+	if filename == "" {
+		return "", fmt.Errorf("filename cannot be empty")
+	}
+
+	// Try current directory first
+	if _, err := os.Stat(filename); err == nil {
+		return filename, nil
+	}
+
+	// Try parent directories (for tests running from subdirectories)
+	paths := []string{
+		filename,               // Current dir
+		"../" + filename,       // One level up
+		"../../" + filename,    // Two levels up (for tests in internal/handlers)
+		"../../../" + filename, // Three levels up
+	}
+
+	for _, path := range paths {
+		if _, err := os.Stat(path); err == nil {
+			return path, nil
+		}
+	}
+
+	return "", fmt.Errorf("file not found: %s (searched: current dir, ../, ../../, ../../../)", filename)
+}