refactor(tools): modularize CLI with urfave/cli/v2, externalise template metadata

- Split monolithic build-preview.go into modular packages:
  tools.go (entry), cli/ (list/create/delete/preview commands),
  config/ (JSON loading), preview/ (rendering engine, funcs, locale)
- Extract all template metadata into data/templates_config.json
  as single source of truth — no hardcoded template data in Go
- Replace hand-rolled CLI parsing with github.com/urfave/cli/v2
- Add sensible defaults for --folder (../themes) and --config
  (./data/templates_config.json) — most commands now run bare
- Add type-coercing gt/lt/ge/le template funcs for JSON float64
- Fix AppUrl to gitea.com matching official template convention
- rendered.js now exports __REGISTRY__ and __PARAMS__ alongside
  __RENDERED__ — preview/index.html reads all from single source
- Cross-validate all 11 template data contexts against official
  Gitea source (services/mailer/*.go + templates/mail/*.tmpl)
- Update all docs (README, AGENTS, CONTRIBUTING, 5 languages)
  to reflect new CLI workflow with create scaffolding command

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/cli/create.go

@@ -0,0 +1,86 @@
+package cli
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/urfave/cli/v2"
+
+	"gitea-mail-templates/tools/config"
+)
+
+// CreateCommand returns the "create" subcommand.
+func CreateCommand() *cli.Command {
+	return &cli.Command{
+		Name:      "create",
+		Usage:     "Create one or more new theme style directories",
+		UsageText: "go run . create [--folder <themes-dir>] [--config <config-file>] <style-name>...",
+		Flags: []cli.Flag{
+			&cli.StringFlag{
+				Name:  "folder",
+				Value: "../themes",
+				Usage: "Path to the themes directory",
+			},
+			&cli.StringFlag{
+				Name:  "config",
+				Value: "./data/templates_config.json",
+				Usage: "Path to templates_config.json",
+			},
+		},
+		Action: runCreate,
+	}
+}
+
+func runCreate(c *cli.Context) error {
+	names := c.Args().Slice()
+	if len(names) == 0 {
+		return fmt.Errorf("at least one style name is required")
+	}
+
+	folder := c.String("folder")
+	cfg, err := config.Load(c.String("config"))
+	if err != nil {
+		return err
+	}
+
+	for _, styleName := range names {
+		styleDir := filepath.Join(folder, styleName)
+
+		if info, err := os.Stat(styleDir); err == nil {
+			if info.IsDir() {
+				fmt.Printf("  [skip] '%s' already exists\n", styleName)
+				continue
+			}
+			return fmt.Errorf("'%s' exists but is not a directory", styleName)
+		}
+
+		created := 0
+		for _, tplCfg := range cfg.Templates {
+			tmplPath := filepath.Join(styleDir, tplCfg.PathStr())
+			dir := filepath.Dir(tmplPath)
+
+			if err := os.MkdirAll(dir, 0755); err != nil {
+				return fmt.Errorf("cannot create directory %s: %w", dir, err)
+			}
+
+			placeholder := fmt.Sprintf("<!DOCTYPE html>\n<html>\n<head>\n"+
+				"  <meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\" />\n"+
+				"  <meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0\" />\n"+
+				"  <title>{{.locale.Tr \"mail.%s.title\"}}</title>\n"+
+				"  <style type=\"text/css\">\n    /* TODO: Add your custom styles here */\n"+
+				"  </style>\n</head>\n<body style=\"margin:0;padding:0;\">\n"+
+				"  <!-- TODO: Add your custom template design for: %s -->\n"+
+				"</body>\n</html>\n", tplCfg.Name, tplCfg.Desc)
+
+			if err := os.WriteFile(tmplPath, []byte(placeholder), 0644); err != nil {
+				return fmt.Errorf("cannot write %s: %w", tmplPath, err)
+			}
+			created++
+		}
+
+		fmt.Printf("  Created style '%s' with %d template files\n", styleName, created)
+	}
+
+	return nil
+}