fix: ABS proxy via backend to bypass CORS + settings button layout

- Add /api/abs-proxy/* endpoint in the backend that forwards all ABS API
  calls server-side (X-ABS-URL + X-ABS-Token headers). Browser never
  touches ABS directly for API calls — no CORS issue.
- abs.js routes through proxy when backendUrl is set; falls back to direct
  if no backend (requires user to configure CORS on ABS server)
- Audio streaming still uses direct <audio src> (HTML media bypasses CORS)
- Settings: status text above buttons, buttons full-width flex row

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -544,12 +544,10 @@
           <label>Bibliothek</label>
           <select id="abs-lib-select" onchange="absSetLibrary(this.value)" style="width:100%;padding:10px 12px;background:var(--bg2);border:1px solid var(--border2);border-radius:var(--radius-sm);color:var(--text);font-family:var(--font);font-size:14px;outline:none"></select>
         </div>
-        <div class="settings-row" style="margin-top:8px;border-radius:9px">
-          <span id="abs-s-status" style="font-size:13px;color:var(--green)"></span>
-          <div style="display:flex;gap:8px">
-            <button class="btn-outline" style="padding:6px 12px;font-size:12px;color:var(--danger);border-color:rgba(224,85,85,0.3)" onclick="absDisconnect()">Trennen</button>
-            <button class="btn-outline" id="abs-s-connect-btn" style="padding:6px 12px;font-size:12px" onclick="absConnectSettings()">Verbinden</button>
-          </div>
+        <span id="abs-s-status" style="font-size:12px;color:var(--green);display:block;margin-bottom:8px;min-height:16px"></span>
+        <div style="display:flex;gap:8px;margin-top:2px">
+          <button class="btn-outline" style="flex:1;padding:9px;font-size:13px;color:var(--danger);border-color:rgba(224,85,85,0.3)" onclick="absDisconnect()">Trennen</button>
+          <button class="btn-outline" id="abs-s-connect-btn" style="flex:1;padding:9px;font-size:13px;font-weight:700" onclick="absConnectSettings()">Verbinden</button>
         </div>
       </div>
       <div class="settings-section">