Add Resource and Prompt discovery

[gauthierpiarrette]

Problem
MCP servers expose three primitives: Tools, Resources, and Prompts. mcpplay currently only discovers and displays Tools. Servers that expose Resources (file contents, API responses) or Prompts (reusable templates) appear incomplete in the playground.

Proposed solution

• Call resources/list and prompts/list during session initialization
• Add sidebar sections or tabs for Resources and Prompts alongside Tools
• Resources: show URI, name, description, mime type. Allow reading resource content inline.
• Prompts: show name, description, arguments. Allow filling arguments and getting the prompt result.

Alternatives considered
Only showing a count/badge for resources and prompts without interaction. Full interaction is better since it matches the "playground" concept — users should be able to exercise all server capabilities.