fix(dropzone): replace broken degruyter upload icon with inline SVG

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

frontend/src/routes/+page.svelte

@@ -350,12 +350,21 @@ $effect(() => {
 			onclick={() => fileInput.click()}
 			onkeydown={(e) => e.key === 'Enter' && fileInput.click()}
 		>
-			<img
-				src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Upload/Upload-MD.svg"
-				alt=""
-				aria-hidden="true"
+			<svg
+				xmlns="http://www.w3.org/2000/svg"
 				class="h-4 w-4 shrink-0 opacity-50"
-			/>
+				viewBox="0 0 24 24"
+				fill="none"
+				stroke="currentColor"
+				stroke-width="2"
+				stroke-linecap="round"
+				stroke-linejoin="round"
+				aria-hidden="true"
+			>
+				<path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4" />
+				<polyline points="17 8 12 3 7 8" />
+				<line x1="12" y1="3" x2="12" y2="15" />
+			</svg>
 			<span class="font-sans font-medium">
 				{isUploading ? '…' : m.upload_drop_hint()}
 			</span>