M17: add interactive base converter

[pgousdal]

M17 — Interactive base converter

Adds the first EduNumbers interactive learning tool.

• converts integer input from bases 2–36
• uses BigInt for exact large-integer handling
• shows binary, octal, decimal, hexadecimal and base-36 representations together
• rejects invalid digits explicitly
• supports signed integers
• includes automated parser tests, including a 64-bit maximum unsigned pattern
• runs the interactive test in the normal CI qualification
• publishes the tool with GitHub Pages
• links it from the Pages landing page

No external JavaScript framework or runtime dependency is required in the browser.