feat: Tor hidden service, PDF paid details, subaddress placeholder

- Tor onion: mc6wfeaqc7oijgdcudrr5zsotmwok3jzk3tu2uezzyjisn7nzzjjizyd.onion
- Onion link in footer and README
- PDF "BEZAHLT" block shows TX hash + date in second line
- Address placeholder 8... (encourages subaddress usage)

index.html

@@ -20,7 +20,7 @@
     <div class="card">
       <div class="field">
         <label for="addr" data-i18n="label_addr">XMR Address</label>
-        <input type="text" id="addr" data-i18n-placeholder="placeholder_addr" placeholder="4..." spellcheck="false" autocomplete="off">
+        <input type="text" id="addr" data-i18n-placeholder="placeholder_addr" placeholder="8..." spellcheck="false" autocomplete="off">
       </div>
 
       <div class="field">
@@ -102,7 +102,7 @@
   </main>
 
   <footer>
-    <p data-i18n-html="footer">Open Source &middot; No Backend &middot; No KYC &middot; <a href="https://gitea.schmidt.eco/schmidt1024/xmrpay.link" target="_blank">Source</a></p>
+    <p data-i18n-html="footer">Open Source &middot; No Backend &middot; No KYC &middot; <a href="https://gitea.schmidt.eco/schmidt1024/xmrpay.link" target="_blank">Source</a> &middot; <a href="http://mc6wfeaqc7oijgdcudrr5zsotmwok3jzk3tu2uezzyjisn7nzzjjizyd.onion" title="Tor Hidden Service">Onion</a></p>
   </footer>
 
   <div class="lang-picker" id="langPicker">