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).
