Fix slug auto-generation to preserve existing slugs on title edits

[Copilot]

Problem

The slug field was automatically regenerating from the post title on every edit, which is detrimental to SEO because it breaks existing URLs and causes 404 errors for previously indexed content. This behavior differs from WordPress and other CMS platforms where slugs are set once during creation and preserved on subsequent edits.

Root Cause

The SlugComponent.tsx contained a useEffect hook that would update the slug whenever the title field changed, regardless of whether it was a new or existing post. While the backend formatSlugHook had the correct logic to only generate slugs for new posts, the frontend component was overriding this behavior.

Solution

Modified the slug component to implement WordPress-like behavior by:

1. Tracking document state using a hasSeenNonEmptySlug ref to distinguish between new and existing documents
2. Preserving existing slugs by only auto-generating when:
   • The document has never had a slug (new document), OR
   • The current slug is empty (user manually cleared it)
3. Maintaining existing functionality for manual slug editing and lock/unlock behavior

Example Behavior

Before (problematic):

1. Create post with title "My First Post" → slug: "my-first-post" ✓
2. Edit title to "My Updated Post" → slug: "my-updated-post" ❌ (breaks SEO)

After (fixed):

1. Create post with title "My First Post" → slug: "my-first-post" ✓
2. Edit title to "My Updated Post" → slug: "my-first-post" ✓ (preserved)

Testing

The fix has been validated through comprehensive test scenarios covering:

• New post creation with auto-slug generation
• Existing post title edits preserving slugs
• Manual slug clearing and regeneration
• Lock/unlock functionality
• Edge cases with empty titles and slugs

All existing functionality remains intact while fixing the core SEO issue.

Original prompt

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This section details on the original issue you should resolve

<issue_title>bug: slug changes on every post's title edit</issue_title>
<issue_description>Changing slugs is highly detrimental to SEO because the old slug will return a 404 error. In WordPress, when you first set a title, it will be used as the slug, and subsequent changes will not modify the slug.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #5

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