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/App.svelte

@@ -157,7 +157,13 @@
     </Sidebar.Content>
 
     <Sidebar.Footer>
-      <Button variant="ghost" size="sm" class="justify-start gap-2" onclick={() => (drawerOpen = true)}>
+      <Button
+        variant="ghost"
+        size="sm"
+        class="justify-start gap-2"
+        onclick={() => (drawerOpen = true)}
+        title="Chat over the current page without navigating away"
+      >
         <PanelRightIcon />
         <span>Chat drawer</span>
       </Button>