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/src/lib/components/IconButton.svelte

@@ -0,0 +1,26 @@
+<script lang="ts">
+	import MdiIcon from './MdiIcon.svelte';
+
+	let { icon, title = '', onclick, disabled = false, variant = 'default', size = 16, class: className = '' } = $props<{
+		icon: string;
+		title?: string;
+		onclick?: (e: MouseEvent) => void;
+		disabled?: boolean;
+		variant?: 'default' | 'danger' | 'success';
+		size?: number;
+		class?: string;
+	}>();
+
+	const variantClasses = {
+		default: 'text-[var(--color-muted-foreground)] hover:text-[var(--color-foreground)] hover:bg-[var(--color-muted)]',
+		danger: 'text-[var(--color-muted-foreground)] hover:text-[var(--color-destructive)] hover:bg-[var(--color-error-bg)]',
+		success: 'text-[var(--color-muted-foreground)] hover:text-[var(--color-success-fg)] hover:bg-[var(--color-success-bg)]',
+	};
+</script>
+
+<button type="button" {title} {onclick} {disabled}
+	class="inline-flex items-center justify-center w-7 h-7 rounded-md transition-colors
+		disabled:opacity-40 disabled:pointer-events-none {variantClasses[variant]} {className}"
+>
+	<MdiIcon name={icon} {size} />
+</button>