Preview-pane guest pages (Streamlit's traceback "Ask Google" / "Ask …"
buttons, plain `<a target="_blank">` anchors) could not open anything: the
`<webview>` has no `allowpopups`, so Chromium drops the popup before any
handler runs. Opening from `setWindowOpenHandler` is banned
(GHSA-9f4c-93c8-jc8g, window-open-policy.ts), so this adds an explicit
click bridge instead:
- main.ts installs a guest preload through `will-attach-webview`, keyed on
the `persist:hermes-preview` partition only.
- The preload forwards a clicked `_blank` anchor's resolved href to the
host renderer via `ipcRenderer.sendToHost`; it opens nothing itself.
- PreviewPane admits the URL and routes it through the existing audited
`hermes:openExternal` IPC.
Salvaged from PR #112959 (squash of its two commits).
Fixes#112941
b08ec00a70