fix(ui): implement UI review findings — a11y, IA, and consistency fixes

Fixes the reviewed gaps: keyboard-inaccessible delete controls (SessionRail,
Entities row), case-sensitive entity filter, two competing entity-detail
navigation patterns (standardize on EntitySheet), non-clickable Overview KPI
cards, a bare button bypassing the shared Button component, inconsistent
blur-only vs live filtering, and an unenforced sanitization assumption on
search snippet HTML (now using the already-present dompurify dependency).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/utils.ts

@@ -21,6 +21,16 @@ export function relativeTime(iso: string | null | undefined): string {
 	return `${d}d ago`;
 }
 
+// debounce wraps fn so rapid calls (e.g. keystrokes in a filter input)
+// collapse into one invocation after `wait`ms of silence.
+export function debounce<T extends (...args: never[]) => void>(fn: T, wait = 300): T {
+	let timer: ReturnType<typeof setTimeout> | undefined;
+	return ((...args: Parameters<T>) => {
+		clearTimeout(timer);
+		timer = setTimeout(() => fn(...args), wait);
+	}) as T;
+}
+
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export type WithoutChild<T> = T extends { child?: any } ? Omit<T, "child"> : T;
 // eslint-disable-next-line @typescript-eslint/no-explicit-any