perf(showcase): cap main viewer image width at 2400 via display mode

- New OptimizationMode.DISPLAY (width=2400 for images, full quality for
  videos) and getDisplayUrl(); the in-modal main viewer renders at most
  ~1200 CSS px wide, so full-size originals wasted 50-70% bandwidth
- Main viewer and adjacent prefetch use display URLs; the full-size
  media viewer keeps using getShowcaseUrl for original quality

static/js/components/shared/showcase/ShowcaseView.js

@@ -22,7 +22,7 @@ import {
 } from './MediaUtils.js';
 import { generateMetadataPanel } from './MetadataPanel.js';
 import { generateImageWrapper, generateVideoWrapper } from './MediaRenderers.js';
-import { getShowcaseUrl, getGalleryThumbnailUrl } from '../../../utils/civitaiUtils.js';
+import { getShowcaseUrl, getDisplayUrl, getGalleryThumbnailUrl } from '../../../utils/civitaiUtils.js';
 import { openMediaViewer } from '../MediaViewer.js';
 import { escapeAttribute } from '../utils.js';
 
@@ -311,8 +311,9 @@ function renderMediaItem(img, index, exampleFiles) {
                   originalRemoteUrl.endsWith('.mp4') || originalRemoteUrl.endsWith('.webm');
     const mediaType = isVideo ? 'video' : 'image';
 
-    // Optimize CivitAI URLs for showcase display (full quality)
+    // Optimize CivitAI URLs for in-modal display (images capped at width=2400;
-    const remoteUrl = getShowcaseUrl(originalRemoteUrl, mediaType);
+    // the full-size media viewer uses getShowcaseUrl separately)
+    const remoteUrl = getDisplayUrl(originalRemoteUrl, mediaType);
 
     const localUrl = localFile ? localFile.path : '';
 
@@ -466,7 +467,9 @@ function prefetchAdjacentMedia() {
         const isVideo = img.url.endsWith('.mp4') || img.url.endsWith('.webm');
         if (isVideo) return;
 
-        const url = getShowcaseUrl(img.url, 'image');
+        // Must match the main viewer's URL (display mode) or the warmed
+        // cache entry is never used
+        const url = getDisplayUrl(img.url, 'image');
         if (prefetchedUrls.has(url)) return;
         prefetchedUrls.add(url);
 

static/js/utils/civitaiUtils.js

@@ -9,6 +9,9 @@
 export const OptimizationMode = {
     /** Full quality for showcase/display - uses /optimized=true only */
     SHOWCASE: 'showcase',
+    /** In-modal display - caps image width at 2400 (covers the ~1200 CSS px
+     * main viewer at DPR 2); videos stay full quality */
+    DISPLAY: 'display',
     /** Thumbnail size for cards - uses /width=450,optimized=true */
     THUMBNAIL: 'thumbnail',
     /** Small thumbnails for the showcase gallery strip (72px display) - uses /width=160,optimized=true */
@@ -97,15 +100,20 @@ export function rewriteCivitaiUrl(sourceUrl, mediaType = null, mode = Optimizati
         }
 
         // Determine replacement based on mode and media type
+        const isVideo = Boolean(mediaType && mediaType.toLowerCase() === 'video');
         let replacement;
         if (mode === OptimizationMode.SHOWCASE) {
             // Full quality for showcase - no width restriction
             replacement = '/optimized=true';
+        } else if (mode === OptimizationMode.DISPLAY) {
+            // Display mode caps image width for in-modal viewing; videos stay
+            // full quality (CDN transcoding costs more than it saves here)
+            replacement = isVideo ? '/optimized=true' : '/width=2400,optimized=true';
         } else {
             // Thumbnail modes with width restriction
             const width = mode === OptimizationMode.GALLERY_THUMBNAIL ? 160 : 450;
             replacement = `/width=${width},optimized=true`;
-            if (mediaType && mediaType.toLowerCase() === 'video') {
+            if (isVideo) {
                 replacement = `/transcode=true,width=${width},optimized=true`;
             }
         }
@@ -153,6 +161,19 @@ export function getShowcaseUrl(url, type = 'image') {
     return getOptimizedUrl(url, type, OptimizationMode.SHOWCASE);
 }
 
+/**
+ * Get display-optimized URL for the in-modal main viewer (images capped at
+ * width=2400; videos full quality). Use getShowcaseUrl for full-size viewing
+ * (e.g. the media viewer overlay)
+ *
+ * @param {string} url - Original URL
+ * @param {string} type - Media type ("image" or "video")
+ * @returns {string} - Optimized URL for in-modal display
+ */
+export function getDisplayUrl(url, type = 'image') {
+    return getOptimizedUrl(url, type, OptimizationMode.DISPLAY);
+}
+
 /**
  * Get thumbnail-optimized URL (width=450)
  * 

tests/frontend/utils/civitaiUtils.test.js

@@ -8,6 +8,7 @@ import {
     rewriteCivitaiUrl,
     getOptimizedUrl,
     getShowcaseUrl,
+    getDisplayUrl,
     getThumbnailUrl,
     getGalleryThumbnailUrl,
     extractCivitaiImageId,
@@ -22,6 +23,7 @@ describe('civitaiUtils', () => {
     describe('OptimizationMode', () => {
         it('should have correct mode values', () => {
             expect(OptimizationMode.SHOWCASE).toBe('showcase');
+            expect(OptimizationMode.DISPLAY).toBe('display');
             expect(OptimizationMode.THUMBNAIL).toBe('thumbnail');
             expect(OptimizationMode.GALLERY_THUMBNAIL).toBe('gallery-thumbnail');
         });
@@ -125,6 +127,22 @@ describe('civitaiUtils', () => {
             expect(rewritten).toBe('https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/transcode=true,width=160,optimized=true/12345.mp4');
         });
 
+        it('should rewrite image URLs with /original=true for display mode (width=2400)', () => {
+            const originalUrl = 'https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/original=true/12345.jpeg';
+            const [rewritten, wasRewritten] = rewriteCivitaiUrl(originalUrl, 'image', OptimizationMode.DISPLAY);
+
+            expect(wasRewritten).toBe(true);
+            expect(rewritten).toBe('https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/width=2400,optimized=true/12345.jpeg');
+        });
+
+        it('should keep videos full quality in display mode (no transcode/width)', () => {
+            const originalUrl = 'https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/original=true/12345.mp4';
+            const [rewritten, wasRewritten] = rewriteCivitaiUrl(originalUrl, 'video', OptimizationMode.DISPLAY);
+
+            expect(wasRewritten).toBe(true);
+            expect(rewritten).toBe('https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/optimized=true/12345.mp4');
+        });
+
         it('should not rewrite URLs without /original=true', () => {
             const originalUrl = 'https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/width=450/12345.jpeg';
             const [rewritten, wasRewritten] = rewriteCivitaiUrl(originalUrl, 'image', OptimizationMode.THUMBNAIL);
@@ -250,6 +268,22 @@ describe('civitaiUtils', () => {
         });
     });
 
+    describe('getDisplayUrl', () => {
+        it('should return display-optimized URL (width=2400) for images', () => {
+            const originalUrl = 'https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/original=true/12345.jpeg';
+            const displayUrl = getDisplayUrl(originalUrl, 'image');
+
+            expect(displayUrl).toBe('https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/width=2400,optimized=true/12345.jpeg');
+        });
+
+        it('should keep videos full quality', () => {
+            const originalUrl = 'https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/original=true/12345.mp4';
+            const displayUrl = getDisplayUrl(originalUrl, 'video');
+
+            expect(displayUrl).toBe('https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/optimized=true/12345.mp4');
+        });
+    });
+
     describe('getGalleryThumbnailUrl', () => {
         it('should return gallery-thumbnail-optimized URL (width=160)', () => {
             const originalUrl = 'https://image.civitai.com/xG1nkqKTMzGDvpLrqFT7WA/abc123/original=true/12345.jpeg';