Fix CSP-blocked inline onclick handlers

- Remove onclick from API key recovery modal Copy button (templates/pages/index.html)
- Event listener already exists in app.js (line 295)
- Remove onclick from server-rendered API key display (src/index.ts line 207)
- Remove onclick from billing success page Copy button (src/routes/billing.ts line 181)
- Create public/copy-helper.js to handle all [data-copy] elements via external JS
- All copy functionality now CSP-compliant (script-src 'self')

templates/pages/index.html

@@ -614,7 +614,7 @@ html, body {
       </div>
       <div style="background:var(--bg);border:1px solid var(--accent);border-radius:8px;padding:14px;font-family:monospace;font-size:0.82rem;word-break:break-all;margin:16px 0;position:relative;">
         <span id="recoveredKeyText"></span>
-        <button onclick="copyRecoveredKey()" id="copyRecoveredBtn" style="position:absolute;top:8px;right:8px;background:var(--accent);color:var(--bg);border:none;border-radius:4px;padding:4px 12px;cursor:pointer;font-size:0.8rem;">Copy</button>
+        <button id="copyRecoveredBtn" style="position:absolute;top:8px;right:8px;background:var(--accent);color:var(--bg);border:none;border-radius:4px;padding:4px 12px;cursor:pointer;font-size:0.8rem;">Copy</button>
       </div>
       <p style="margin-top:20px;color:var(--muted);font-size:0.9rem;"><a href="/docs">Read the docs →</a></p>
     </div>