fix(a11y): add aria-modal to ConfirmDialog for older AT

NVDA+Chrome <2022 and VoiceOver iOS <16 need explicit aria-modal="true";
showModal() implicit modal semantics are not enough for older AT. One-line
patch benefits all dialog uses.

Refs #781
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/shared/primitives/ConfirmDialog.svelte

@@ -19,6 +19,7 @@ $effect(() => {
 <dialog
 	bind:this={dialogEl}
 	class="m-auto w-full max-w-sm rounded-sm border border-line bg-surface p-6 shadow-lg backdrop:bg-black/50"
+	aria-modal="true"
 	aria-labelledby="confirm-title"
 	oncancel={(e) => {
 		e.preventDefault();