msg b64: base64-encode display prefix in d-field to avoid pipe/quote breaking parse

b64_encode/decode (URL-safe A-Za-z0-9-_) in chat_core_priv.h
on_media_registered encodes msg_data before full_data
on_msg_inserted/chat_core_attachment_download decode base_filename
android_jni_bridge decodes prefix for UI display

tools/chatgui-android/jni_bridge/android_jni_bridge.c

@@ -413,6 +413,19 @@ char* utun_bridge_get_messages_json(const char* channel_id, int limit) {
             const char* cts = strstr((const char*)dptr, "\"ct\":\"");
             if (cts) { cts += 6; char* cte = strchr((char*)cts, '"'); if (cte) { size_t tlc = (size_t)(cte - cts); if (tlc < sizeof(ct)) { memcpy(ct, cts, tlc); ct[tlc] = '\0'; } } }
         }
+        /* base64-decode display prefix for media messages (encoded in on_media_registered) */
+        if (strcmp(ct, "text/plain") != 0 && txt[0]) {
+            char* pipe = strchr(txt, '|');
+            if (pipe) {
+                size_t prefix_len = (size_t)(pipe - txt);
+                uint8_t dec[1024]; int declen = (int)b64_decode(txt, prefix_len, dec, sizeof(dec));
+                if (declen > 0 && declen < 2000) {
+                    size_t suffix_len = strlen(pipe);
+                    memmove(txt + declen, pipe, suffix_len + 1);
+                    memcpy(txt, dec, (size_t)declen);
+                }
+            }
+        }
         char* esc_txt = json_escape_alloc(txt);
         char* esc_ct  = json_escape_alloc(ct);
         int is_out = (node_id == (int64_t)my_id) ? 1 : 0;