Add diagnostic tools discovery and DNS/HTTP checks

[1337lean]

Summary

• Added new diagnostic back-end helpers for DNS resolver comparison, email DNS health checks, and HTTP security header inspection.
• Extended the tool API route to validate tool-specific request options, route the new diagnostics, and rate-limit/dedupe them appropriately.
• Added UI updates for a tool launcher, quick-access links, related tools, and diagnostic result presentation styles.
• Introduced shared diagnostic types and tests covering DNS diagnostics, HTTP security, request parsing, and tool discovery.

Testing

• Not run (not requested).
• Added/updated unit tests for DNS diagnostics, HTTP security checks, request option parsing, and discovery behavior.
• Manual checks implied by the UI changes: launcher display, tool navigation, and diagnostic result rendering.