refactor: Have ExperienceSection/EducationSection return full section object to avoid type assertions

## Summary

In `EditorContent.tsx`, the `onUpdate` callbacks for `ExperienceSection` and `EducationSection` reconstruct the section object and use `as Section` type assertions:

```typescript
// Lines 319-378 in EditorContent.tsx
onUpdate={(updatedContent) => {
const updatedSection = {
...section,
content: updatedContent,
} as Section; // Type assertion bypasses safety checks
handleSectionUpdate(index, updatedSection);
}}

This pattern bypasses TypeScript's type safety checks.

Proposed Solution

Refactor child components (ExperienceSection, EducationSection) to:
1. Accept the full section object as a prop (not just content)
2. Return the entire updated section object in onUpdate
3. Eliminate the need for parent-side reconstruction and type assertions

Before

// Parent (EditorContent.tsx)
{
const updatedSection = { ...section, content: updatedContent } as Section;
handleSectionUpdate(index, updatedSection);
}}
/>

After

// Parent (EditorContent.tsx)
{
handleSectionUpdate(index, updatedSection);
}}
/>

Files to Update

- src/components/editor/EditorContent.tsx
- src/components/ExperienceSection.tsx
- src/components/EducationSection.tsx
- Related test files