perf(app): switch map thumbnails to _800.webp variant

_400.webp pixelates very quickly when the user zooms into the
force-graph canvas — the 22-graph-unit nodes blow up to hundreds of
on-screen pixels and the 400px source can't keep up. Bump the variant
to _800.webp: stays crisp under typical zoom-in, total payload still
only ~5 MB for 312 specs (vs ~15 MB for full-size originals).

Refs #5646

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MarkusNeusinger

app/src/pages/MapPage.helpers.test.ts

@@ -160,15 +160,15 @@ describe('buildKNNLinks', () => {
 
 
 describe('selectMapThumbUrl', () => {
-  it('returns the _400.webp variant for the current theme', () => {
+  it('returns the _800.webp variant for the current theme', () => {
     const s = spec('a', null);
-    expect(selectMapThumbUrl(s, true)).toBe('https://example.com/a-dark_400.webp');
-    expect(selectMapThumbUrl(s, false)).toBe('https://example.com/a-light_400.webp');
+    expect(selectMapThumbUrl(s, true)).toBe('https://example.com/a-dark_800.webp');
+    expect(selectMapThumbUrl(s, false)).toBe('https://example.com/a-light_800.webp');
   });
 
   it('falls back to the other theme variant when the preferred URL is missing', () => {
     const s: SpecMapItem = { ...spec('a', null), preview_url_dark: null };
-    expect(selectMapThumbUrl(s, true)).toBe('https://example.com/a-light_400.webp');
+    expect(selectMapThumbUrl(s, true)).toBe('https://example.com/a-light_800.webp');
   });
 
   it('returns null when no preview URLs at all', () => {

app/src/pages/MapPage.helpers.ts

@@ -144,10 +144,10 @@ export function buildKNNLinks(
 
 /**
  * Pick the best thumbnail URL for the current theme and downsize it to the
- * `_400.webp` variant produced by the responsive-image pipeline. Map nodes
- * render at ~22 px, so even 400 px is overkill — but 400 is the smallest
- * pipeline-baked variant. Going from full-size (~40–100 KB) to _400.webp
- * (~6 KB) cuts the 312-thumbnail payload from ~15 MB to ~2 MB.
+ * `_800.webp` variant produced by the responsive-image pipeline. _800 stays
+ * crisp under typical zoom-in (the smaller _400 variant pixelates quickly),
+ * while keeping the 312-thumbnail payload at ~5 MB total instead of the
+ * ~15 MB the full-size originals would cost.
  *
  * Falls back to the original full-size URL if the convention can't be
  * applied (e.g. URL doesn't end in `.png`).
@@ -156,7 +156,7 @@ export function selectMapThumbUrl(spec: SpecMapItem, isDark: boolean): string |
   const full = selectPreviewUrl(spec, isDark);
   if (!full) return null;
   if (!full.endsWith('.png')) return full;
-  return full.replace(/\.png$/, '_400.webp');
+  return full.replace(/\.png$/, '_800.webp');
 }
 
 /**