feat: modernize codebase and complete API surface

- Update module path from prologic/pastebin to taigrr/pastebin
- Replace go.rice with stdlib embed for templates and static assets
- Replace httprouter with stdlib net/http ServeMux (Go 1.22+)
- Replace namsral/flag with cobra + fang for CLI argument parsing
- Fix inverted TLS insecure logic in client (was negating the flag)
- Close HTTP response body in client
- Use base64 URLEncoding instead of StdEncoding for paste IDs
- Add comprehensive handler tests for all endpoints
- Add client package tests
- Add utils tests for RandomString
- Remove templates.go (replaced by embed)
- Remove unused dependencies (go-metrics, stats, logger, httprouter, go.rice)

utils_test.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRandomStringLength(t *testing.T) {
+	for _, length := range []int{1, 4, 8, 16, 32} {
+		result := RandomString(length)
+		assert.Len(t, result, length, "RandomString(%d) should return string of length %d", length, length)
+	}
+}
+
+func TestRandomStringUniqueness(t *testing.T) {
+	seen := make(map[string]bool)
+	for range 100 {
+		result := RandomString(pasteIDLength)
+		require.False(t, seen[result], "RandomString produced duplicate: %s", result)
+		seen[result] = true
+	}
+}