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/preview_cmd.go

@@ -0,0 +1,85 @@
+package cli
+
+import (
+	"fmt"
+	"log"
+	"path/filepath"
+
+	"github.com/urfave/cli/v2"
+
+	"gitea-mail-templates/tools/config"
+	"gitea-mail-templates/tools/preview"
+)
+
+// PreviewCommand returns the "preview" subcommand.
+func PreviewCommand() *cli.Command {
+	return &cli.Command{
+		Name:  "preview",
+		Usage: "Pre-render mail templates for preview/testing",
+		UsageText: `go run . preview [--folder <themes-dir>] [--config <config-file>] [--output <output.js>] all
+   go run . preview [--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",
+			},
+			&cli.StringFlag{
+				Name:  "output",
+				Usage: "Path to output rendered.js (default: derived from --folder)",
+			},
+		},
+		Action: runPreview,
+	}
+}
+
+func runPreview(c *cli.Context) error {
+	folder := c.String("folder")
+	configPath := c.String("config")
+
+	outputPath := c.String("output")
+	if outputPath == "" {
+		outputPath = filepath.Join(filepath.Dir(folder), "preview", "rendered.js")
+	}
+
+	names := c.Args().Slice()
+	if len(names) == 0 {
+		return fmt.Errorf("specify style names or 'all' (e.g. go run . preview all)")
+	}
+
+	cfg, err := config.Load(configPath)
+	if err != nil {
+		return err
+	}
+
+	var themeFilter map[string]bool
+	if len(names) == 1 && names[0] == "all" {
+		themeFilter = nil
+	} else {
+		themeFilter = make(map[string]bool)
+		for _, name := range names {
+			themeFilter[name] = true
+		}
+	}
+
+	result := preview.RenderAll(folder, cfg, themeFilter)
+	if result == nil {
+		return fmt.Errorf("no results produced")
+	}
+
+	if err := preview.WriteRenderedJS(result, outputPath); err != nil {
+		return err
+	}
+
+	preview.PrintDetailedSummary(result, folder, cfg)
+
+	log.Printf("Wrote %d themes x %d templates to %s",
+		len(result.Summaries), len(cfg.Templates), outputPath)
+
+	return nil
+}