fix(web): show feed images again, widen the grid, soften covers

- Drop the service worker's image cache: its cross-origin fetch() is bound
  by the CSP's connect-src 'self', so every cover failed and fell back to
  the green placeholder (in the grid and the reader).
- Grid fills the width with as many ~20rem columns as fit (max 2400px),
  now that the sidebar is gone.
- Covers fade in once loaded and get a light bottom shade.
- Placeholder cover shows a centered initial instead of the feed name
  that overlapped the feed chip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NsXUyMEGFFGVRcHNVfcBLh

web/vite.config.ts

@@ -67,15 +67,9 @@ export default defineConfig({
               cacheableResponse: { statuses: [200] },
             },
           },
-          {
-            urlPattern: ({ request }) => request.destination === 'image',
-            handler: 'StaleWhileRevalidate',
-            options: {
-              cacheName: 'images',
-              expiration: { maxEntries: 400, maxAgeSeconds: 60 * 60 * 24 * 30, purgeOnQuotaError: true },
-              cacheableResponse: { statuses: [0, 200] },
-            },
-          },
+          // No image route: feed images are cross-origin, and a fetch() from
+          // the worker is bound by the CSP's connect-src 'self', so it would
+          // fail every cover. The browser's HTTP cache handles them instead.
           {
             urlPattern: ({ request }) => request.destination === 'font',
             handler: 'CacheFirst',