Add test data box to template editor for live preview

Enter a JS snippet returning a fields object (e.g. return { resistance: '10k' })
to preview the formatter against synthetic data instead of the first real component.
Both the formatter and test data textareas update the preview on input.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/templates.html

@@ -184,6 +184,10 @@
 				<label for="tmpl-formatter">Formatter <span class="label-hint">(JS arrow function, return null to skip)</span></label>
 				<textarea id="tmpl-formatter" rows="8" class="code-input" placeholder="(c) => {&#10;  const r = c.fields?.resistance;&#10;  if (!r) return null;&#10;  return `Resistor ${r}`;&#10;}"></textarea>
 			</div>
+			<div class="form-row">
+				<label for="tmpl-test-data">Test data <span class="label-hint">(optional — return a fields object to preview against)</span></label>
+				<textarea id="tmpl-test-data" rows="3" class="code-input" placeholder="return { resistance: '10k', mounting_tech: 'PTH' }"></textarea>
+			</div>
 			<div class="tmpl-preview-row">
 				<span class="label-hint">Preview:</span>
 				<span id="tmpl-preview" class="tmpl-preview-value">—</span>