Open WebUI: Same-origin XSS to account takeover via terminal port-preview iframe hardcoding allow-same-origin

## Summary Any authenticated user with access to a shared terminal server could get script of their choosing to run in the Open WebUI origin itself. The in-app port preview rendered the content of a previewed port in an iframe whose sandbox always granted `allow-same-origin` alongside `allow-scripts`, and that content is served from a path on the application's own origin, so the sandbox provided no isolation at all. Script served on a previewed port could read the victim's session token and take over the account. ## Preconditions - At least one terminal server configured by an admin (`TERMINAL_SERVER_CONNECTIONS`, empty by default) and reachable by both the attacker and the victim. Deployments with no terminal server configured are not affected. - The attacker needs a normal authenticated account with access to that terminal connection, no admin rights, plus the ability to start a process listening on a port there. - Personal terminals a user configures for themselves are not affected. Those carry an external URL, so the previewed document is cross-origin and the sandbox held. - The victim has to open the port list and click the attacker's port. - `TERMINAL_PROXY_HEADERS` unset and no `CONTENT_SECURITY_POLICY` set, which are the defaults. An operator who had already set a restrictive Content-Security-Policy through either was not exposed, since those headers apply to the proxied response. - The `iframeSandboxAllowSameOrigin` user setting is off by default, but the affected br