Remote browser pages inherit Chromium origin zoom

If you zoom a site in Orca's local browser, then open that same site from a paired client, the remote page comes in huge. Cmd/0 on the remote pane does nothing.

## Details

Chromium stores page zoom per origin per partition (`HostZoomMap`). Local panes reassert zoom on every `dom-ready`. Offscreen guests (headless/`orca serve`) and screencast sessions never did, so leftover origin zoom from a local tab leaks into the remote stream.

## Reproduction

1. On the host Mac, open Orca and create a local browser tab for `https://github.com/electron/electron` (any origin works).
2. Focus the page and press Cmd++ a few times until the page is clearly zoomed (about 150–170% is enough).
3. Pair a remote client: another desktop, the web client, or a phone. The host can also be `orca serve`.
4. On the remote client, open a new browser tab to the same URL.
5. The remote page is zoomed in. GitHub's header overflows. You see less of the page. Cmd/0 on the remote pane does not reset it.

Expected: a newly opened remote page starts at 100%, same as a new local tab.

## Platforms

Reproduced on macOS with an isolated Orca profile. Same partition sharing applies to Linux headless serve and SSH/offscreen guests.

Start by tracing the local panes' dom-ready zoom reassertion and compare it with offscreen guests, headless orca serve, and screencast session initialization. Reproduce the shared-origin case from the listed steps; done means a newly opened remote page starts at 100% and Cmd/0 can reset the remote pane.

Written by the indexing model from the issue text.