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/list.go

@@ -0,0 +1,57 @@
+package cli
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+
+	"github.com/urfave/cli/v2"
+
+	"gitea-mail-templates/tools/preview"
+)
+
+// ListCommand returns the "list" subcommand.
+func ListCommand() *cli.Command {
+	return &cli.Command{
+		Name:      "list",
+		Usage:     "List available theme styles in the target folder",
+		UsageText: "go run . list [--folder <themes-dir>]",
+		Flags: []cli.Flag{
+			&cli.StringFlag{
+				Name:  "folder",
+				Value: "../themes",
+				Usage: "Path to the themes directory",
+			},
+		},
+		Action: runList,
+	}
+}
+
+func runList(c *cli.Context) error {
+	folder := c.String("folder")
+
+	themes, err := preview.DiscoverThemes(folder)
+	if err != nil {
+		return err
+	}
+
+	absDir, _ := filepath.Abs(folder)
+	fmt.Printf("Available styles in '%s' (%d):\n", filepath.Base(absDir), len(themes))
+
+	sort.Strings(themes)
+	for _, t := range themes {
+		count := 0
+		filepath.Walk(filepath.Join(folder, t), func(path string, info os.FileInfo, err error) error {
+			if err != nil {
+				return nil
+			}
+			if filepath.Ext(path) == ".tmpl" {
+				count++
+			}
+			return nil
+		})
+		fmt.Printf("  %-16s (%d .tmpl files)\n", t, count)
+	}
+	return nil
+}