Add Post Creation UI

[cameronhotchkies]

Build the React UI for creating new markdown posts. Completes the Micro Steel Thread.

✅ Acceptance Criteria

• Logged-in user sees "New Post" button in navigation
• Clicking "New Post" → navigates to create form
• Form has title and markdown body fields
• Submitting valid form → creates post, redirects to post detail
• Newly created post appears at top of feed
• Logged-out user does not see "New Post" button
• Attempting to access create form while logged out redirects to login

Internal Coding

MR2

• Create post form component
• Submit flow with API integration
• Auth-gated "New Post" button in navigation