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>

docs/images/README.md

@@ -11,7 +11,7 @@ neon.png       mono.png        terra.png    ink.png      aurora.png
 
 ## How to Capture
 
-1. Run `go run ./tools/build-preview.go` from the project root
+1. Run `cd tools && go run . preview all` from the project root
 2. Open `preview/index.html` in a browser
 3. For each style, select the "Activate Account" template and "Modern" client mode
 4. Take a screenshot of the rendered email (600px width recommended)