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/preview/locale.go

@@ -0,0 +1,96 @@
+package preview
+
+import (
+	"fmt"
+	"html/template"
+	"strings"
+)
+
+// Locale provides English fallback translations, matching Gitea's locale.Tr interface.
+type Locale struct{}
+
+// Tr translates a key and substitutes positional %s/%d arguments.
+// HTML arguments are preserved; plain strings are HTML-escaped.
+func (l Locale) Tr(key string, args ...any) template.HTML {
+	s := LocaleEN[key]
+	if s == "" {
+		s = key
+	}
+	for _, arg := range args {
+		var val string
+		if h, ok := arg.(template.HTML); ok {
+			val = string(h) // already safe HTML — don't double-escape
+		} else {
+			val = template.HTMLEscapeString(fmt.Sprint(arg))
+		}
+		s = strings.Replace(s, "%s", val, 1)
+		s = strings.Replace(s, "%d", fmt.Sprint(arg), 1)
+	}
+	return template.HTML(s)
+}
+
+// TrN picks singular/plural by count, then delegates to Tr.
+func (l Locale) TrN(count any, singular, plural string, args ...any) template.HTML {
+	n := 0
+	switch v := count.(type) {
+	case int:
+		n = v
+	case int64:
+		n = int(v)
+	case float64:
+		n = int(v)
+	default:
+		n = 1
+	}
+	key := plural
+	if n == 1 {
+		key = singular
+	}
+	return l.Tr(key, args...)
+}
+
+// LocaleEN holds all English translation strings used by Gitea mail templates.
+var LocaleEN = map[string]string{
+	"mail.activate_account.title":         "Activate your %s account",
+	"mail.hi_user_x":                      "Hi %s,",
+	"mail.activate_account.text_1":        "Welcome to %s! Please activate your account by clicking the button below.",
+	"mail.activate_account.text_2":        "This activation link will expire in %s.",
+	"mail.activate_email.title":           "Verify your email address for %s",
+	"mail.activate_email.text":            "Please verify your email address. The link will expire in %s.",
+	"mail.register_notify.title":          "Welcome to %s, %s",
+	"mail.register_notify.text_1":         "Your account on %s has been created successfully.",
+	"mail.register_notify.text_2":         "Your username is: %s",
+	"mail.register_notify.text_3":         "If you need to set a password, visit %s.",
+	"mail.reset_password.title":           "Reset your %s password",
+	"mail.reset_password.text":            "Click the link below to reset your password. It will expire in %s.",
+	"mail.link_not_working_do_paste":      "If the button doesn't work, copy and paste this link into your browser:",
+	"mail.team_invite.text_1":             "%s has invited you to join the team %s in the organization %s.",
+	"mail.team_invite.text_2":             "Click the button below to accept the invitation.",
+	"mail.team_invite.text_3":             "This invitation was sent to %s.",
+	"mail.repo.collaborator.added.text":   "You have been added as a collaborator on the repository",
+	"mail.view_it_on":                     "View it on %s",
+	"mail.repo.transfer.body":             "The repository %s has been transferred to you.",
+	"mail.release.new.text":               "%s published a new release %s in the repository %s.",
+	"mail.release.title":                  "Release: %s",
+	"mail.release.note":                   "Release Notes:",
+	"mail.release.downloads":              "Downloads",
+	"mail.release.download.zip":           "Source Code (ZIP)",
+	"mail.release.download.targz":         "Source Code (TAR.GZ)",
+	"mail.issue_assigned.pull":            "%s assigned pull request %s to you in the repository %s.",
+	"mail.issue_assigned.issue":           "%s assigned issue %s to you in the repository %s.",
+	"mail.issue.x_mentioned_you":          "%s mentioned you.",
+	"mail.issue.action.close":             "%s closed issue #%d.",
+	"mail.issue.action.reopen":            "%s reopened issue #%d.",
+	"mail.issue.action.merge":             "%s merged pull request #%d into %s.",
+	"mail.issue.action.approve":           "%s approved this change.",
+	"mail.issue.action.reject":            "%s requested changes.",
+	"mail.issue.action.review":            "%s reviewed this change.",
+	"mail.issue.action.review_dismissed":  "%s dismissed the review from %s.",
+	"mail.issue.action.ready_for_review":  "%s marked this as ready for review.",
+	"mail.issue.action.new":               "%s created issue #%d.",
+	"mail.issue.action.force_push":        "%s force-pushed the %s branch from %s to %s.",
+	"mail.issue.action.push_1":            "%s pushed %d commit to %s.",
+	"mail.issue.action.push_n":            "%s pushed %d commits to %s.",
+	"mail.issue.in_tree_path":             "In %s:",
+	"mail.reply":                          "Reply to this email",
+}