style: replace all inline SVGs with Lucide icons

Swap every inline SVG across Header, SearchBar, NoteCard, NoteEditor,
Checklist, and ImageUpload for Lucide components. Use Palette for the
color picker icon instead of the paintbrush. Zero inline SVGs remain.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/components/ImageUpload.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+	import XIcon from 'lucide-svelte/icons/x';
 	import type { Attachment } from '$lib/types/index.js';
 
 	interface Props {
@@ -69,9 +70,7 @@
 						aria-label="Remove attachment"
 						data-testid="remove-attachment"
 					>
-						<svg class="h-3 w-3" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-							<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12" />
-						</svg>
+						<XIcon class="h-3 w-3" />
 					</button>
 				</div>
 			{/each}