feat: add upload page UI (moodboard + SNS feed)

* feat: add upload page initial UI

* feat: add moodboard and SNS feed upload layouts

* fix: improve mobile upload page layout

src/routes/upload/+page.svelte

@@ -0,0 +1,65 @@
+<script>
+	import Header from '$lib/components/ui/Header.svelte';
+	import Artwork from '$lib/components/ui/Artwork/Artwork.svelte';
+	import MoodboardGrid from '$lib/components/ui/upload/MoodboardGrid.svelte';
+	import SnsFeedUpload from '$lib/components/ui/upload/SnsFeedUpload.svelte';
+
+	// "Build Moodboard" is selected by default in the design
+	let mode = $state('moodboard');
+</script>
+
+<!--
+	On desktop the split layout is locked to the viewport height so the left
+	artwork stays put while switching modes. The right panel is a full-bleed
+	upload canvas with the mode toggle floating over it.
+-->
+<div
+	class="flex h-dvh flex-col overflow-x-hidden bg-surface text-ink lg:h-screen lg:overflow-hidden"
+>
+	<Header step={3} total={6} />
+
+	<main class="flex min-h-0 flex-1 flex-col lg:flex-row">
+		<Artwork />
+
+		<!-- Right panel: full-bleed workspace + floating tab switch -->
+		<section
+			class="relative flex min-h-0 flex-1 flex-col pb-[4.75rem] lg:overflow-hidden lg:pb-0"
+		>
+			{#if mode === 'moodboard'}
+				<MoodboardGrid />
+			{:else}
+				<SnsFeedUpload />
+			{/if}
+
+			<!-- full-width on mobile; centered pill on desktop -->
+			<div
+				class="fixed right-0 bottom-0 left-0 z-20 px-4 pb-5 lg:absolute lg:right-auto lg:bottom-8 lg:left-1/2 lg:w-auto lg:-translate-x-1/2 lg:px-0 lg:pb-0"
+			>
+				<div
+					class="flex w-full items-center rounded-full bg-surface/95 p-1.5 shadow-xl ring-1 ring-black/5 backdrop-blur lg:w-auto"
+				>
+					<button
+						type="button"
+						onclick={() => (mode = 'sns')}
+						class={[
+							'flex-1 rounded-full px-4 py-2.5 text-center text-sm whitespace-nowrap transition-colors lg:flex-none lg:px-5',
+							mode === 'sns' ? 'bg-pill text-surface' : 'text-muted hover:text-ink'
+						]}
+					>
+						Upload SNS Feed
+					</button>
+					<button
+						type="button"
+						onclick={() => (mode = 'moodboard')}
+						class={[
+							'flex-1 rounded-full px-4 py-2.5 text-center text-sm whitespace-nowrap transition-colors lg:flex-none lg:px-5',
+							mode === 'moodboard' ? 'bg-pill text-surface' : 'text-muted hover:text-ink'
+						]}
+					>
+						Build Moodboard
+					</button>
+				</div>
+			</div>
+		</section>
+	</main>
+</div>