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/pages/Entities.svelte

@@ -44,13 +44,14 @@
 
   const types = $derived(Array.from(new Set(entities.map((e) => e.type))).sort())
 
-  const filtered = $derived(
-    entities.filter((e) => {
+  const filtered = $derived.by(() => {
+    const q = query.trim().toLowerCase()
+    return entities.filter((e) => {
       if (typeFilter !== 'all' && e.type !== typeFilter) return false
-      if (query && !e.slug.includes(query) && !e.name.includes(query)) return false
+      if (q && !e.slug.toLowerCase().includes(q) && !e.name.toLowerCase().includes(q)) return false
       return true
     })
-  )
+  })
 
   function stateVariant(state?: string | null): 'default' | 'secondary' | 'outline' {
     if (!state) return 'outline'
@@ -116,7 +117,10 @@
           {#each filtered as entity (entity.id)}
             <Table.Row
               class="cursor-pointer"
+              role="button"
+              tabindex={0}
               onclick={() => openEntity(entity.slug)}
+              onkeydown={(e) => { if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); openEntity(entity.slug) } }}
             >
               <Table.Cell class="font-mono text-xs">{entity.slug}</Table.Cell>
               <Table.Cell><Badge variant="outline">{entity.type}</Badge></Table.Cell>