a11y: add aria-hidden to decorative SVGs throughout index.html

[comnam90]

Summary

The map UI contains ~15+ inline SVG icons that are decorative (convey no information beyond their adjacent text or context) but are missing aria-hidden=\"true\". This means screen readers may announce them, producing noise for assistive technology users.

Affected locations

All in layouts/index.html:

Line	Element	Fix
597	Search input magnifier icon	aria-hidden="true"
620	Multiselect chevron	aria-hidden="true"
644	Region count pin icon	aria-hidden="true"
652	Reset filters X icon	aria-hidden="true"
659, 662, 665	Theme toggle icons (iconSystem, iconLight, iconDark)	aria-hidden="true"
671	Info button question-mark icon	aria-hidden="true"
684	Loading spinner SVG	aria-hidden="true"
695	Error state warning icon	aria-hidden="true"
718	Close info panel X icon	aria-hidden="true"
726, 763, 772, 781, 790	Info panel section icons and external link arrows	aria-hidden="true"

Acceptance criteria

• All decorative SVGs in layouts/index.html have aria-hidden="true"
• No SVG that is the sole label for a control is hidden (e.g. icon-only buttons must keep their aria-label)
• npm run test:ui passes

Notes

This is a pure HTML attribute addition — no logic changes required. Good scope for a first contribution.