fix(admin): preserve panel scroll on rebuild (#615)

[CaYatur]

Summary

• Fixes Admin UI: panel scroll jumps on mode / layer / type changes #615: Admin #sections scroll jumped on mode / layers / layer-type (and other render() rebuilds).
• Keep capturing prevScroll, but re-apply after layout with double requestAnimationFrame, clamp to max scroll, and drop stale restores when render() runs again quickly.

Test plan

Linux box verification (2026-09-26, CaYatur / xvfb host)

• Source: admin.js has sectionsScrollRestoreGen, double requestAnimationFrame, Math.min(prevScroll, max), isConnected guard
• Unit: tests/admin-sections-scroll.test.js (source contract + behavioral) — node --test → 5 pass (follow-up test(admin): cover #sections scroll restore (#615/#616) #618)
• GUI: Open Admin, scroll mid-panel on Scene (not run — no interactive Admin click-through on this Linux box)
• GUI: Change visualizer/background mode — scroll stays
• GUI: Expand/enable layers and change a layer type — scroll stays
• GUI: Rapid toggles do not snap to top
• GUI: MilkDrop list scroll behavior unchanged

[CaYatur]

Linux box verification — 2026-09-26 (Europe/Istanbul)

Verified on this host (CaYatur / gh auth):

• Source contract in src/admin/admin.js: sectionsScrollRestoreGen, double requestAnimationFrame, Math.min(prevScroll, max), isConnected guard — present on main.
• New unit test tests/admin-sections-scroll.test.js: node --test → 5 pass (follow-up PR test(admin): cover #sections scroll restore (#615/#616) #618).

Not done: interactive Admin GUI click-through (Scene scroll / mode / layers / rapid toggles / MilkDrop list). Those GUI boxes stay unchecked — unit + source cover the algorithm on Linux without Electron UI driving.