🍱 Add Deno serve QR server example

[AnandChowdhary]

Summary

• Add examples/server.ts, a Deno.serve example that returns generated QR codes as GIF responses.
• Document how to run the server and use long/short query parameters.
• Add integration tests for GIF responses, HEAD requests, method rejection, and query-param fallback behavior.

Test plan

• /tmp/deno-bin/deno lint examples/server.ts examples/server_test.ts
• /tmp/deno-bin/deno test --allow-read --coverage=coverage
• /tmp/deno-bin/deno coverage coverage
• Local HTTP probe against deno run --allow-net examples/server.ts for GET, HEAD, and POST

Fixes #17