Unify slug field behavior across content forms #97

The slug UI is inconsistent between add and edit forms for posts, tags, authors, and other content. Some forms show a non-editable prefix, while others expose the complete slug, including the prefix, as editable text.

Goal:
Provide one consistent slug editing experience across all content types.

Checklist:

• Define a shared slug field component
• Separate non-editable prefixes from editable slug values
• Use the same behavior in add and edit forms
• Apply the pattern to posts, tags, authors, and other slug-based models
• Validate uniqueness and allowed characters
• Preserve existing slugs during editing unless explicitly changed
• Add UI and backend tests