feat: upload button in AppShell sidebar — click or drag to upload screenshots

- Dashed border button at bottom of sidebar nav
- Click to open file picker
- Drag files onto the bottom rail area to upload
- Shows "Uploading..." then "Saved!" for 2 seconds
- Files save to platform/screenshots/ with timestamp names
- Works from any page in the Atelier shell

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

frontend-v2/src/lib/components/layout/AppShell.svelte

@@ -8,7 +8,6 @@
 		CircleDot,
 		Compass,
 		Dumbbell,
-		FolderOpen,
 		Landmark,
 		LibraryBig,
 		Menu,
@@ -16,7 +15,7 @@
 		Search,
 		Settings2,
 		SquareCheckBig,
-		Tag,
+		Upload,
 		X
 	} from '@lucide/svelte';
 
@@ -52,6 +51,33 @@
 	);
 
 	let mobileNavOpen = $state(false);
+	let uploadInput: HTMLInputElement;
+	let uploadStatus = $state<'' | 'uploading' | 'done'>('');
+
+	async function handleUpload(file: File) {
+		uploadStatus = 'uploading';
+		const fd = new FormData();
+		fd.append('file', file);
+		try {
+			await fetch('/upload', { method: 'POST', body: fd, credentials: 'include' });
+			uploadStatus = 'done';
+			setTimeout(() => uploadStatus = '', 2000);
+		} catch {
+			uploadStatus = '';
+		}
+	}
+
+	function onUploadInput(e: Event) {
+		const input = e.target as HTMLInputElement;
+		if (input.files?.[0]) handleUpload(input.files[0]);
+		input.value = '';
+	}
+
+	function onRailDrop(e: DragEvent) {
+		e.preventDefault();
+		const file = e.dataTransfer?.files[0];
+		if (file) handleUpload(file);
+	}
 
 	// Brain sidebar sub-items
 	interface BrainFolder { id: string; name: string; item_count: number; }
@@ -116,7 +142,13 @@
 			</nav>
 		</div>
 
-		<div class="rail-bottom">
+		<!-- svelte-ignore a11y_no_static_element_interactions -->
+		<div class="rail-bottom" ondragover={(e) => e.preventDefault()} ondrop={onRailDrop}>
+			<button class="rail-upload" onclick={() => uploadInput?.click()}>
+				<Upload size={14} strokeWidth={1.8} />
+				<span>{uploadStatus === 'uploading' ? 'Uploading...' : uploadStatus === 'done' ? 'Saved!' : 'Upload screenshot'}</span>
+			</button>
+			<input bind:this={uploadInput} type="file" accept="image/*,.pdf" onchange={onUploadInput} hidden />
 			<div class="rail-date">
 				<CalendarDays size={14} strokeWidth={1.8} />
 				<span>{new Intl.DateTimeFormat('en-US', { month: 'short', day: 'numeric' }).format(new Date())}</span>
@@ -383,6 +415,26 @@
 		border-top: 1px solid var(--shell-line);
 	}
 
+	.rail-upload {
+		display: flex;
+		align-items: center;
+		gap: 8px;
+		padding: 9px 12px;
+		border-radius: 999px;
+		border: 1px dashed var(--shell-line);
+		background: none;
+		color: var(--shell-muted);
+		font-size: 0.78rem;
+		font-family: inherit;
+		cursor: pointer;
+		transition: all 160ms;
+	}
+	.rail-upload:hover {
+		border-color: var(--shell-ink);
+		color: var(--shell-ink);
+		background: rgba(255,255,255,0.3);
+	}
+
 	.rail-date {
 		display: inline-flex;
 		align-items: center;