fix(telegram): remove 'default' user fallback — resolve user dynamically

- botUserID no longer captured once at startup (was 'default' if no user yet)
- resolveBotUser() reads first registered user from DB on demand:
  * called on every /start (handles: registered after bot launch)
  * called before every AI message (handles mid-session registration)
- If no user registered: clear English error 'No account found. Please register on the web UI first'
- start.sh: fix set_env_var appending without newline (token was concatenated to prev line)

start.sh

@@ -115,6 +115,10 @@ set_env_var() {
             sed -i "s|^${var_name}=.*|${var_name}=${var_value}|" .env
         fi
     else
+        # Ensure .env ends with a newline before appending
+        if [ -s ".env" ] && [ "$(tail -c1 .env | wc -l)" -eq 0 ]; then
+            echo "" >> .env
+        fi
         echo "${var_name}=${var_value}" >> .env
     fi
 }