fix: cover_url bug, add save-cover endpoint, fix svelte-check CI failure

- Fix handlers_image.go: cover_url now uses /api/cover/novelfire.net/{slug} (was 'local')
- Add POST /api/admin/image-gen/save-cover: persists pre-generated base64 directly to
  MinIO via PutCover without re-calling Cloudflare AI
- Add SvelteKit proxy route api/admin/image-gen/save-cover/+server.ts
- Update UI saveAsCover(): send existing image_b64 to save-cover instead of re-generating
- Run npm run paraglide to compile admin_nav_image_gen message; force-add gitignored files
- svelte-check: 0 errors, 21 warnings (all pre-existing)

ui/src/routes/admin/image-gen/+page.svelte

@@ -219,28 +219,12 @@
 		saveSuccess = false;
 
 		try {
-			const payload = {
-				prompt: prompt.trim(),
-				model: result.model,
-				type: 'cover',
-				slug: result.slug,
-				num_steps: numSteps,
-				guidance,
-				strength,
-				width,
-				height,
-				save_to_cover: true
-			};
-
-			// Re-generate with save_to_cover=true (backend saves atomically)
-			// Alternatively, we could add a separate save endpoint.
-			// For now we pass the same prompt + model to re-generate and save.
-			// TODO: A lighter approach would be a dedicated save endpoint that accepts
-			// the base64 payload. For now re-gen is acceptable given admin-only usage.
-			const res = await fetch('/api/admin/image-gen', {
+			// Extract the raw base64 from the data URL (data:<mime>;base64,<b64>)
+			const b64 = result.imageSrc.split(',')[1];
+			const res = await fetch('/api/admin/image-gen/save-cover', {
 				method: 'POST',
 				headers: { 'Content-Type': 'application/json' },
-				body: JSON.stringify(payload)
+				body: JSON.stringify({ slug: result.slug, image_b64: b64 })
 			});
 			const body = await res.json().catch(() => ({}));
 			if (!res.ok) {