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>

README.md

@@ -85,16 +85,16 @@ A live preview tool is included to browse all styles and email types without dep
 First, generate the preview data (requires Go):
 
 ```bash
-go run ./tools/build-preview.go
+cd tools && go run . preview all
 ```
 
 Then open `preview/index.html` directly in a browser. No server required.
 
-> `preview/rendered.js` is generated and git-ignored. Re-run the command after modifying templates.
+> `preview/rendered.js` is generated and git-ignored. Re-run `cd tools && go run . preview all` after modifying templates. Use `cd tools && go run .` to see all commands (list, create, delete, preview). Most flags have sensible defaults — just `go run . create <name>` or `go run . preview all` works out of the box.
 
 ### Features
 
-- Theme switcher — toggle between all 5 visual styles
+- Theme switcher — toggle between all 10 visual styles
 - Template switcher — browse all 11 email types
 - Client simulation — Modern, Gmail (no `<style>`), Outlook Desktop, Raw source
 - Viewport toggle — Desktop (1386x780) / Mobile (390x780)
@@ -112,8 +112,12 @@ gitea-mail-templates/
 ├── preview/              # Live preview SPA
 │   ├── index.html        # Style/template/client/viewport switcher
 │   └── rendered.js       # Pre-rendered templates (generated by tools/)
-├── tools/                # Build tooling
-│   ├── build-preview.go  # Go build script — renders all templates
+├── tools/                # Modular CLI tooling
+│   ├── tools.go          #   Main entry point
+│   ├── cli/              #   CLI subcommands (list, create, delete, preview)
+│   ├── config/           #   Config types and templates_config.json loading
+│   ├── data/             #   templates_config.json — single source of truth
+│   ├── preview/          #   Template rendering engine
 │   └── go.mod
 ├── docs/                 # Multi-language documentation
 ├── AGENTS.md             # AI agent guidance