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/config/config.go

@@ -0,0 +1,85 @@
+package config
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// TemplateConfig holds metadata for a single mail template type.
+type TemplateConfig struct {
+	Name   string         `json:"name"`
+	Desc   string         `json:"desc"`
+	Path   []string       `json:"path"`
+	Params map[string]any `json:"params"`
+}
+
+// TemplatesConfig is the root structure of templates_config.json.
+type TemplatesConfig struct {
+	Templates map[string]TemplateConfig `json:"templates"`
+}
+
+// Load reads and parses a templates_config.json file.
+func Load(path string) (*TemplatesConfig, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("cannot read config file %s: %w", path, err)
+	}
+
+	var cfg TemplatesConfig
+	if err := json.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("cannot parse config file %s: %w", path, err)
+	}
+
+	if len(cfg.Templates) == 0 {
+		return nil, fmt.Errorf("config file %s contains no template definitions", path)
+	}
+
+	return &cfg, nil
+}
+
+// PathStr returns the file path of a template definition joined with OS separator.
+func (t *TemplateConfig) PathStr() string {
+	return filepath.Join(t.Path...)
+}
+
+// Registry builds a lookup map suitable for serializing into rendered.js
+// as window.__REGISTRY__: maps template ID → {name, desc, path}.
+func Registry(cfg *TemplatesConfig) map[string]map[string]string {
+	reg := make(map[string]map[string]string, len(cfg.Templates))
+	for id, t := range cfg.Templates {
+		reg[id] = map[string]string{
+			"name": t.Name,
+			"desc": t.Desc,
+			"path": strings.Join(t.Path, "/"),
+		}
+	}
+	return reg
+}
+
+// FlattenParams converts nested template params into a flat dot-notation map
+// suitable for the preview panel display in rendered.js.
+func FlattenParams(params map[string]any) map[string]string {
+	flat := make(map[string]string)
+	flatten("", params, flat)
+	return flat
+}
+
+func flatten(prefix string, val any, out map[string]string) {
+	switch v := val.(type) {
+	case map[string]any:
+		for k, inner := range v {
+			key := k
+			if prefix != "" {
+				key = prefix + "." + k
+			}
+			flatten(key, inner, out)
+		}
+	case []any:
+		out[prefix] = fmt.Sprintf("[%d items]", len(v))
+	default:
+		out[prefix] = fmt.Sprintf("%v", v)
+	}
+}