relay: file attachments, outbox, CC-self, chat view, mobile layout

Compose: attach files (encrypted, hex-named, linked via relay_uid),
CC-self checkbox tags copy with recipient name. Outbox section shows
sent messages with delete. Chat view: session-based threads/history/
send endpoints, split layout with sidebar, read receipts, 5s poll.
Mobile: threads as horizontal scroll bar above messages. Fix chat
input pushed offscreen by adding min-height:0 on flex container.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/index.php

@@ -900,7 +900,11 @@ if (str_starts_with($route, '/api/')) {
                         }
                         if (strlen($raw) >= 17) {
                             $decrypted = openssl_decrypt(substr($raw, 16), 'aes-256-cbc', $encKey, OPENSSL_RAW_DATA, substr($raw, 0, 16));
-                            if ($decrypted !== false) file_put_contents($dest, $decrypted);
+                            if ($decrypted !== false) {
+                                $fileBytes = base64_decode($decrypted);
+                                if ($fileBytes !== false) $decrypted = $fileBytes;
+                                file_put_contents($dest, $decrypted);
+                            }
                         }
                     }
                 }