fix: replace target=_blank links with shell bridge open_url

[sanity]

Problem

freenet/freenet-core is removing allow-popups-to-escape-sandbox from the gateway iframe sandbox to fix a security vulnerability where a malicious web app can open a popup that escapes the sandbox and gains full localhost:7509 origin access -- enabling it to interact with other web apps, forge delegate operations, and bypass the new permission prompt system.

Without allow-popups-to-escape-sandbox, popups opened from the iframe inherit the sandbox (opaque origin). This breaks external links in River because the target website sees Origin: null in CORS requests, causing failures.

Solution

The gateway shell page now supports an open_url message via the existing postMessage bridge. Instead of <a target="_blank">, web apps send:

parent.postMessage({
    __freenet_shell__: true,
    type: 'open_url',
    url: 'https://example.com'
}, '*');

The shell page validates the URL (must be https:, must not be localhost) and opens it with window.open(url, '_blank', 'noopener'), giving the popup proper origin without sandbox inheritance.

Changes Needed in River

In ui/src/components/conversation.rs, the add_target_blank_to_links function (around line 406) currently adds target="_blank" to all <a> tags in rendered messages. This needs to be changed so that link clicks are intercepted and routed through the shell bridge instead.

Options:

1. Add an onclick handler that calls parent.postMessage(...) and prevents default navigation
2. Or use the freenet-stdlib helper (if one is added) for shell bridge URL opening

Blocked By

This is blocked by the freenet-core PR that removes allow-popups-to-escape-sandbox and adds the open_url shell bridge handler. External links will break in River once that PR is released until this change is made.

Priority

High -- must be done immediately after the freenet-core release that removes allow-popups-to-escape-sandbox.

[AI-assisted - Claude]