Fix [object Object] in Variables button + allow preview of system templates

- Escape {{ }} in Svelte template (use {'{{ }}'} string expression)
- Allow _get() to access system templates (user_id=0) for preview

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

frontend/src/routes/template-configs/+page.svelte

@@ -133,7 +133,7 @@
 								<label class="text-xs text-[var(--color-muted-foreground)]">{t(`templateConfig.${slot.label}`)}</label>
 								{#if varsRef[slot.key]}
 									<button type="button" onclick={() => showVarsFor = slot.key}
-										class="text-xs text-[var(--color-muted-foreground)] hover:underline">{{ }} Variables</button>
+										class="text-xs text-[var(--color-muted-foreground)] hover:underline">{'{{ }}'} Variables</button>
 								{/if}
 							</div>
 							{#if (slot.rows || 2) > 2}