fix(network): unpin the pair modal from the chat bar and cap the glasses QR
Constrain `.link-modal` to the band between the facet bar and chat bar and let `.link-modal-box` scroll internally, so `#link-pair-cancel` is no longer painted under the fixed chat bar.
Cap the `is-glasses` QR at `min(100%, 360px)` instead of `100%`, so the modal no longer overflows the viewport.
Add `DEVICE_LABEL_PLACEHOLDER_COMPUTER` and set the `#link-device-label` placeholder from the selected presentation mode in `setPairCopy()`. The placeholder seam runs before a pair request exists because `setPresentationMode()` calls `setPairCopy()` ahead of `renderPairPresentation()` and its `!currentPair` guard.
Co-Authored-By: OpenAI Codex <codex@openai.com>