feat(skill-editor): implement StartTabContent with modular component structure

Refactor StartTabContent into separate components following Figma design specs:
- ActionCard: reusable card with icon, title, description
- SectionHeader: title/xl-semi-bold header with description
- CreateImportSection: 3-column grid layout for Create/Import cards
- SkillTemplatesSection: templates area with placeholder

Align styles with Figma: 3-col grid, 16px title, proper spacing and padding.
Add i18n translations for all user-facing text (en-US, zh-Hans).

web/app/components/workflow/skill/start-tab/section-header.tsx

@@ -0,0 +1,27 @@
+'use client'
+
+import type { FC } from 'react'
+import { memo } from 'react'
+
+type SectionHeaderProps = {
+  title: string
+  description: string
+}
+
+const SectionHeader: FC<SectionHeaderProps> = ({
+  title,
+  description,
+}) => {
+  return (
+    <header className="mb-3 flex flex-col gap-0.5">
+      <h2 className="title-xl-semi-bold text-text-primary">
+        {title}
+      </h2>
+      <p className="system-xs-regular text-text-tertiary">
+        {description}
+      </p>
+    </header>
+  )
+}
+
+export default memo(SectionHeader)