feat(internal): add platform detection and auth packages

- internal/platform: cross-platform path detection, 6 tests
- internal/auth: JWT generation/validation + bcrypt, 8 tests
- 89 total tests, build + vet clean

backend/go.mod

@@ -4,4 +4,8 @@ go 1.25.0
 
 require github.com/joho/godotenv v1.5.1
 
-require github.com/lib/pq v1.12.3
+require (
+	github.com/golang-jwt/jwt/v5 v5.3.1
+	github.com/lib/pq v1.12.3
+	golang.org/x/crypto v0.54.0
+)