fix: update CF AI image gen to use multipart for FLUX.2 models

Cloudflare Workers AI changed the API for flux-2-dev, flux-2-klein-4b,
and flux-2-klein-9b to require multipart/form-data (instead of JSON) and
now returns {"image":"<base64>"} instead of raw PNG bytes.

- Add requiresMultipart() helper for the three FLUX.2 models
- callImageAPI builds multipart body for those models, JSON for others
- Parse {"image":"<base64>"} JSON response; fall back to raw bytes for legacy models
- Use "steps" field name (not "num_steps") in multipart forms per CF docs
- Book page: capture and display actual backend error message instead of blank 'Error'

ui/src/routes/books/[slug]/+page.svelte

@@ -164,6 +164,7 @@
 	let coverPreview = $state<string | null>(null);
 	let coverSaving = $state(false);
 	let coverResult = $state<'saved' | 'error' | ''>('');
+	let coverErrorMsg = $state('');
 	let coverPromptOpen = $state(false);
 
 	function buildCoverPrompt(): string {
@@ -186,6 +187,7 @@
 		coverGenerating = true;
 		coverPreview = null;
 		coverResult = '';
+		coverErrorMsg = '';
 		const promptToUse = coverPrompt.trim() || buildCoverPrompt();
 		try {
 			let res: Response;
@@ -210,22 +212,25 @@
 			} else {
 				res = await fetch('/api/admin/image-gen', {
 					method: 'POST',
-					headers: { 'Content-Type': 'application/json' },
-					body: JSON.stringify({ slug, type: 'cover', prompt: promptToUse })
-				});
-			}
-			if (res.ok) {
-				const d = await res.json();
-				coverPreview = d.image_b64 ? `data:${d.content_type ?? 'image/png'};base64,${d.image_b64}` : null;
-			} else {
-				coverResult = 'error';
-			}
-		} catch {
-			coverResult = 'error';
-		} finally {
-			coverGenerating = false;
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ slug, type: 'cover', prompt: promptToUse })
+			});
 		}
+		if (res.ok) {
+			const d = await res.json();
+			coverPreview = d.image_b64 ? `data:${d.content_type ?? 'image/png'};base64,${d.image_b64}` : null;
+		} else {
+			const d = await res.json().catch(() => ({}));
+			coverErrorMsg = (d as any).error ?? '';
+			coverResult = 'error';
+		}
+	} catch (e: any) {
+		coverErrorMsg = e?.message ?? '';
+		coverResult = 'error';
+	} finally {
+		coverGenerating = false;
 	}
+}
 
 	async function saveCover() {
 		const slug = data.book?.slug;
@@ -258,6 +263,7 @@
 	let chapterCoverGenerating = $state(false);
 	let chapterCoverPreview = $state<string | null>(null);
 	let chapterCoverResult = $state<'saved' | 'error' | ''>('');
+	let chapterCoverErrorMsg = $state('');
 	let chapterCoverSaving = $state(false);
 	let chapterCoverPrompt = $state('');
 
@@ -269,6 +275,7 @@
 		chapterCoverGenerating = true;
 		chapterCoverPreview = null;
 		chapterCoverResult = '';
+		chapterCoverErrorMsg = '';
 		const promptToUse = chapterCoverPrompt.trim() || `Chapter ${n} illustration for "${data.book?.title ?? slug}". Dramatic scene, vivid colors, detailed art, cinematic lighting.`;
 		try {
 			const res = await fetch('/api/admin/image-gen', {
@@ -280,9 +287,12 @@
 				const d = await res.json();
 				chapterCoverPreview = d.image_b64 ? `data:${d.content_type ?? 'image/png'};base64,${d.image_b64}` : null;
 			} else {
+				const d = await res.json().catch(() => ({}));
+				chapterCoverErrorMsg = (d as any).error ?? '';
 				chapterCoverResult = 'error';
 			}
-		} catch {
+		} catch (e: any) {
+			chapterCoverErrorMsg = e?.message ?? '';
 			chapterCoverResult = 'error';
 		} finally {
 			chapterCoverGenerating = false;
@@ -1101,7 +1111,7 @@
 							{m.book_detail_admin_generate()}{coverUseAsRef ? ' (img2img)' : ''}
 						</button>
 						{#if coverResult === 'error'}
-							<span class="text-xs text-(--color-danger)">{m.common_error()}</span>
+							<span class="text-xs text-(--color-danger)">{coverErrorMsg || m.common_error()}</span>
 						{:else if coverResult === 'saved'}
 							<span class="text-xs text-green-400">{m.book_detail_admin_saved()}</span>
 						{/if}
@@ -1159,7 +1169,7 @@
 							{m.book_detail_admin_generate()}
 						</button>
 						{#if chapterCoverResult === 'error'}
-							<span class="text-xs text-(--color-danger)">{m.common_error()}</span>
+							<span class="text-xs text-(--color-danger)">{chapterCoverErrorMsg || m.common_error()}</span>
 						{:else if chapterCoverResult === 'saved'}
 							<span class="text-xs text-green-400">{m.book_detail_admin_saved()}</span>
 						{/if}