fix: keep combobox popovers anchored on scroll (#296)

[BorisTyshkevich]

What & why

Closes #296. Open shared combobox popovers now reposition on capture-phase document scroll events, which keeps their fixed listboxes aligned while the Workbench variable strip or Dashboard filter strip scrolls horizontally. The listener is attached only while a combobox is open and is removed by the common close path.

Validation

• npm test passes, including 100/100/100/100 coverage for src/ui/combobox.ts
• npm run build succeeds
• npm run test:e2e is environment-blocked: all browsers exit with SIGTRAP at launch before loading app code; a forced Chromium reinstall is blocked because cdn.playwright.dev is not allowlisted.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• CHANGELOG.md [Unreleased] updated for the behavior change
• Reconciled affected tracked work; Combobox popover doesn't reposition on horizontal filter-strip scroll (broader exposure after #294) #296 is a standalone inbox fix and does not reshape roadmap Roadmap to 1.0.0 #68