feat: migrate all icons to Lucide React

Replace unicode characters, emoji, and inline SVGs with lucide-react
components across the entire design system and page layer. Update
tests to assert on SVG elements instead of text content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/layout/Sidebar/SidebarTree.tsx

@@ -9,6 +9,7 @@ import {
   type MouseEvent,
 } from 'react'
 import { useNavigate } from 'react-router-dom'
+import { Star, ChevronRight, ChevronDown } from 'lucide-react'
 import styles from './Sidebar.module.css'
 
 // ── Types ────────────────────────────────────────────────────────────────────
@@ -35,22 +36,14 @@ export interface SidebarTreeProps {
   autoRevealPath?: string | null  // when set, auto-expand the parent of the matching node
 }
 
-// ── Star icon SVGs ───────────────────────────────────────────────────────────
+// ── Star icons ───────────────────────────────────────────────────────────────
 
 function StarOutline() {
-  return (
-    <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2">
-      <path d="M12 2l3.09 6.26L22 9.27l-5 4.87 1.18 6.88L12 17.77l-6.18 3.25L7 14.14 2 9.27l6.91-1.01L12 2z" />
-    </svg>
-  )
+  return <Star size={14} />
 }
 
 function StarFilled() {
-  return (
-    <svg width="14" height="14" viewBox="0 0 24 24" fill="currentColor" stroke="currentColor" strokeWidth="2">
-      <path d="M12 2l3.09 6.26L22 9.27l-5 4.87 1.18 6.88L12 17.77l-6.18 3.25L7 14.14 2 9.27l6.91-1.01L12 2z" />
-    </svg>
-  )
+  return <Star size={14} fill="currentColor" />
 }
 
 // ── Persistent expand state ──────────────────────────────────────────────────
@@ -395,7 +388,7 @@ function SidebarTreeRow({
               tabIndex={-1}
               aria-label={isExpanded ? 'Collapse' : 'Expand'}
             >
-              {isExpanded ? '▾' : '▸'}
+              {isExpanded ? <ChevronDown size={12} /> : <ChevronRight size={12} />}
             </button>
           ) : null}
         </span>