Keep the page behind a modal from scrolling on touch

[drudge]

Problem

On iOS Safari the page behind an open modal (e.g. the Insights finding drawer) still scrolls by touch. `html:has(dialog[open]) { overflow: hidden }` covers wheel/trackpad, but iOS ignores root overflow for touch scrolling. A drag on the drawer header or backdrop scrolls the document, the toolbar collapses, and the drawer slides off with the page.

Fix

• app.js: while a `:modal` dialog is open, a non-passive `touchmove` guard calls `preventDefault` unless the touch is inside the modal on an element that can scroll along the gesture's axis. Text fields, selects, and contenteditable keep their drags.
• app.css: `dialog[open] * { overscroll-behavior: contain }`, so scrollers in a modal (e.g. `.query-detail-body`) don't chain to the page when they hit an edge.

I chose this over pinning `` with `position: fixed` because that breaks `position: sticky` (`.mobile-header`, the replica banner) behind centered modals and has to restore the scroll offset afterward.

Tests

• New `TestBrowserModalScrollLock` (`-tags browser`, `scripts/browser/modal-scroll-lock.cjs`) sends real CDP touches on a 390×844 touch viewport. It checks that backdrop drags and drags on a modal part that can't scroll are refused, and that a scroller inside the modal still scrolls and contains overscroll. It fails without the app.js change.
• `TestBrowserNavSwipe` still passes. `mage verify` is green.

No visual change, so there are no before and after screenshots.