De-risk the html{zoom} layout dependency (Safari verification + fallback/guard)

[BorisTyshkevich]

Part of #68 (Roadmap to 1.0.0) — release-blocker.

Problem

The whole UI depends on html { zoom: var(--zoom) } (src/styles.css:3, factor 1.2). Because zoom does not divide viewport units and getBoundingClientRect() is post-zoom, the app hand-corrects everywhere: zoomScale() in src/ui/dom.js, the calc(... / var(--zoom)) viewport panels, and pointer-mapping in editor.js / results.js / schema-detail.js / splitters.js / chart-data.js. There is no @supports (zoom: 1) feature detection and no fallback (no @supports in the stylesheet). On any engine that ignores zoom or diverges in how it interacts with rects/viewport units, the UI renders at the wrong scale and every pointer/caret/drag computation is wrong — a hard failure, not graceful degradation.

zoom is non-standard: Chrome/Edge always supported it, Firefox added it in 2024 (~v126), WebKit/Safari supports it but with known rect/viewport divergences — exactly the seams this app corrects.

Ask

• Verify behavior on WebKit/Safari (depends on the WebKit-e2e issue).
• Either confirm it's correct everywhere, or add a guard: @supports feature-detection with a documented fallback, or move to a non-zoom scaling approach (e.g. rem/root-font scaling or a transform wrapper) if Safari diverges.
• At minimum, fail loudly / degrade gracefully rather than silently mis-scaling on an unsupported engine.

Acceptance

• Confirmed correct on all engines in the support matrix, OR a feature-guard/fallback is in place.
• No silent wrong-scale / wrong-pointer state on an engine that doesn't honor zoom.
• npm test + e2e green; per-file coverage gate holds.

[BorisTyshkevich]

Two-step, per the build order in #68:

1. For 1.0: WebKit-verify the html{zoom} paths (caret/graph/splitter/chart/table/popover-anchor) and add the fallback/guard this issue calls for.
2. Full removal of root zoom is a coordinate-math refactor touching the same paths the CM6 (Adopt CodeMirror 6 behind the EditorPort seam (replace the hand-rolled textarea editor) #21) and GraphSurface (Schema graph: multi-select + group-move of nodes (marquee, Shift-click, ⌘A) with single-step undo #66) rewrites already rewrite — fold it into those rather than a standalone big-bang (avoids touching the editor/graph twice).