Add live template preview (auto-updates as you type)

Preview now piggybacks on the validation debounce call — when the
template renders successfully, the result is shown in a collapsible
<details> section below the editor. Removes the manual Preview button
since it's now automatic. Preview hides when there are errors.

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

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

@@ -35,6 +35,8 @@
 			slotErrors = { ...slotErrors, [slotKey]: '' };
 			slotErrorLines = { ...slotErrorLines, [slotKey]: null };
 			slotErrorTypes = { ...slotErrorTypes, [slotKey]: '' };
+			const { [slotKey]: _, ...rest } = slotPreview;
+			slotPreview = rest;
 			return;
 		}
 
@@ -45,6 +47,13 @@
 				slotErrors = { ...slotErrors, [slotKey]: res.error || '' };
 				slotErrorLines = { ...slotErrorLines, [slotKey]: res.error_line || null };
 				slotErrorTypes = { ...slotErrorTypes, [slotKey]: res.error_type || '' };
+				// Live preview: show rendered result when no error
+				if (res.rendered) {
+					slotPreview = { ...slotPreview, [slotKey]: res.rendered };
+				} else {
+					const { [slotKey]: _, ...rest } = slotPreview;
+					slotPreview = rest;
+				}
 			} catch {
 				// Network error, don't show as template error
 				slotErrors = { ...slotErrors, [slotKey]: '' };
@@ -180,10 +189,6 @@
 							<div class="flex items-center justify-between mb-1">
 								<label class="text-xs text-[var(--color-muted-foreground)]">{t(`templateConfig.${slot.label}`)}</label>
 								<div class="flex items-center gap-2">
-									{#if (slot.rows || 2) > 2}
-										<button type="button" onclick={() => previewSlot(slot.key)}
-											class="text-xs text-[var(--color-muted-foreground)] hover:underline">{t('templateConfig.preview')}</button>
-									{/if}
 									{#if varsRef[slot.key]}
 										<button type="button" onclick={() => showVarsFor = slot.key}
 											class="text-xs text-[var(--color-muted-foreground)] hover:underline">{t('templateConfig.variables')}</button>
@@ -199,10 +204,13 @@
 										<p class="mt-1 text-xs" style="color: var(--color-error-fg);">✕ {t('common.syntaxError')}: {slotErrors[slot.key]}{slotErrorLines[slot.key] ? ` (${t('common.line')} ${slotErrorLines[slot.key]})` : ''}</p>
 									{/if}
 								{/if}
-								{#if slotPreview[slot.key]}
+								{#if slotPreview[slot.key] && !slotErrors[slot.key]}
+									<details class="mt-1">
+										<summary class="text-xs text-[var(--color-muted-foreground)] cursor-pointer hover:underline">{t('templateConfig.preview')}</summary>
 										<div class="mt-1 p-2 bg-[var(--color-muted)] rounded text-sm">
 											<pre class="whitespace-pre-wrap">{slotPreview[slot.key]}</pre>
 										</div>
+									</details>
 								{/if}
 							{:else}
 								<input bind:value={(form as any)[slot.key]}