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/routes/servers/+page.svelte

@@ -9,6 +9,7 @@
 	import IconPicker from '$lib/components/IconPicker.svelte';
 	import MdiIcon from '$lib/components/MdiIcon.svelte';
 	import ConfirmModal from '$lib/components/ConfirmModal.svelte';
+	import IconButton from '$lib/components/IconButton.svelte';
 
 	let servers = $state<any[]>([]);
 	let showForm = $state(false);
@@ -76,7 +77,11 @@
 <PageHeader title={t('servers.title')} description={t('servers.description')}>
 	<button onclick={() => { showForm ? (showForm = false, editing = null) : openNew(); }}
 		class="px-3 py-1.5 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90">
-		{showForm ? t('servers.cancel') : t('servers.addServer')}
+		{#if showForm}
+			{t('servers.cancel')}
+		{:else}
+			<span class="flex items-center gap-1"><MdiIcon name="mdiPlus" size={14} />{t('servers.addServer')}</span>
+		{/if}
 	</button>
 </PageHeader>
 
@@ -136,9 +141,9 @@
 							<p class="text-sm text-[var(--color-muted-foreground)]">{server.url}</p>
 						</div>
 					</div>
-					<div class="flex items-center gap-3">
-						<button onclick={() => edit(server)} class="text-xs text-[var(--color-muted-foreground)] hover:underline">{t('common.edit')}</button>
-						<button onclick={() => startDelete(server)} class="text-xs text-[var(--color-destructive)] hover:underline">{t('servers.delete')}</button>
+					<div class="flex items-center gap-1">
+						<IconButton icon="mdiPencil" title={t('common.edit')} onclick={() => edit(server)} />
+						<IconButton icon="mdiDelete" title={t('common.delete')} onclick={() => startDelete(server)} variant="danger" />
 					</div>
 				</div>
 			</Card>