Jinja2 syntax highlighting + description field + preview toggle

JinjaEditor:
- Custom StreamLanguage parser for Jinja2 syntax highlighting:
  {{ variables }} in blue, {% statements %} in purple, {# comments #} in gray
- Replaced HTML mode (didn't understand Jinja2 syntax)
- Proper monospace font (Consolas/Monaco)

TemplateConfig:
- Added `description` field to model + seed defaults with descriptions
- Description shown on template cards instead of raw template text
- Description input in create/edit form

Preview:
- Toggle behavior: clicking Preview again hides the preview
- Per-slot preview uses preview-raw API (renders current editor content)

i18n: added common.description, templateConfig.descriptionPlaceholder

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/package-lock.json

@@ -9,6 +9,7 @@
 			"version": "0.0.1",
 			"dependencies": {
 				"@codemirror/lang-html": "^6.4.11",
+				"@codemirror/language": "^6.12.2",
 				"@codemirror/state": "^6.6.0",
 				"@codemirror/theme-one-dark": "^6.1.3",
 				"@codemirror/view": "^6.40.0",