revert(web): drop the 32x32 PNG favicon fallback

The SVG media query alone fixes the dark-mode icon for every SVG-capable
browser (Chrome, Edge, Firefox, Safari 26+); the PNG only served Safari
versions before 26, which is a shrinking legacy audience not worth a second
asset, its manifest/link declarations, and the extra test surface. index.html
and the manifest are back to the single SVG icon.

apps/web/index.html

@@ -4,7 +4,6 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <link rel="manifest" href="/manifest.webmanifest" />
-    <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png" />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <title>DeepSeek Harness</title>
   </head>

apps/web/public/manifest.webmanifest

@@ -6,12 +6,6 @@
   "scope": "/",
   "display": "fullscreen",
   "icons": [
-    {
-      "src": "/favicon-32x32.png",
-      "sizes": "32x32",
-      "type": "image/png",
-      "purpose": "any"
-    },
     {
       "src": "/favicon.svg",
       "sizes": "any",

apps/web/tests/pwa-manifest.e2e.ts

@@ -5,17 +5,9 @@ import { expect, it } from 'vitest'
 
 const DIST_ROOT = fileURLToPath(new URL('../dist', import.meta.url))
 
-/** PNG signature per the PNG specification. */
-const PNG_SIGNATURE = [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]
-
 it('ships install metadata with the built web application', async () => {
   const index = await readFile(join(DIST_ROOT, 'index.html'), 'utf8')
   expect(index).toContain('<link rel="manifest" href="/manifest.webmanifest" />')
-  expect(index).toContain('<link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png" />')
-  // The SVG must stay declared after the PNG: the HTML spec selects the last
-  // equally appropriate icon, so SVG-capable browsers get the adaptive SVG
-  // while Safari versions before 26 fall back to the PNG.
-  expect(index.indexOf('/favicon-32x32.png')).toBeLessThan(index.indexOf('/favicon.svg'))
 
   const manifest: unknown = JSON.parse(await readFile(join(DIST_ROOT, 'manifest.webmanifest'), 'utf8'))
   expect(manifest).toEqual({
@@ -25,31 +17,15 @@ it('ships install metadata with the built web application', async () => {
     start_url: '/',
     scope: '/',
     display: 'fullscreen',
-    icons: [
-      {
-        src: '/favicon-32x32.png',
-        sizes: '32x32',
-        type: 'image/png',
-        purpose: 'any',
-      },
-      {
-        src: '/favicon.svg',
-        sizes: 'any',
-        type: 'image/svg+xml',
-        purpose: 'any',
-      },
-    ],
+    icons: [{
+      src: '/favicon.svg',
+      sizes: 'any',
+      type: 'image/svg+xml',
+      purpose: 'any',
+    }],
   })
 })
 
-it('ships the PNG fallback as a valid non-empty 32x32 image', async () => {
-  const png = await readFile(join(DIST_ROOT, 'favicon-32x32.png'))
-  expect([...png.subarray(0, 8)]).toEqual(PNG_SIGNATURE)
-  expect(png.readUInt32BE(16)).toBe(32)
-  expect(png.readUInt32BE(20)).toBe(32)
-  expect(png.length).toBeGreaterThan(0)
-})
-
 it('ships a favicon that switches to a light mark under dark color scheme', async () => {
   const favicon = await readFile(join(DIST_ROOT, 'favicon.svg'), 'utf8')
   // The light fill must live inside the dark-scheme media query, so the icon