Add EIP-6963 multi-wallet picker to the join tools

Both /join-now and /direct-join now discover every injected wallet (Brave,
MetaMask, SafePal, Trust, Coinbase…) via EIP-6963 and, when more than one is
present, show a 'Choose your wallet' picker on connect — instead of blindly
grabbing window.ethereum (which silently hangs when e.g. Brave's built-in wallet
wins the slot). Shared public/rmc-wallet.js; provider resolved on connect and
wallet events bound once after. Falls back to window.ethereum for legacy
single-provider wallets. CSP-safe (6963 icons are data URIs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

private/direct-join.html

@@ -78,4 +78,4 @@
 
   <p class="micro" style="text-align:center">No income is guaranteed. Cryptocurrency and smart-contract participation carry risk. Only use funds you can afford to lose.</p>
 </main>
-<script src="/direct-join.js"></script></body></html>
+<script src="/rmc-wallet.js"></script><script src="/direct-join.js"></script></body></html>