Summary: Prompt Wallet adds pricing page and plans

Why It Matters: This means Prompt Wallet is now charging money for some features.

What Changed:

• A "Pricing" link was added to the main menu.
• A "Pricing" section was added to the page.
• The "Pricing" section shows a "Free" plan and a "Plus" plan.
• The "Free" plan offers 20 saved prompts, public shares, 2 public libraries, and Markdown support.
• The "Plus" plan costs $4.99/month and offers unlimited prompts, shares, libraries, and future custom URL/profile features.
• The detailed descriptions for "Save and organize prompts," "Central," "Organize," "Search," "Full Markdown," "Versioning," and "Share Prompts" sections were removed.
• A new "Platform overview" section was added with summarized features: "Centralize every prompt in one place," "Write with rich Markdown & diagrams," "Track every iteration automatically," and "Share securely with the right people."