feat(brand): split-W PWA icons + favicons (replaces purple-star placeholder)

- New mark: solid "W" glyph color-split left=white / right=yellow over the
  Camogli harbor photo from logo.svg; right half reads as a "V" so the W
  alone communicates "WeVisto" at icon scale where the wordmark is illegible.
- PWA icons (192, 512, apple-touch 180) rendered full-bleed; maskable
  variant shrinks the W to the inner 65% so circle/squircle launcher masks
  crop sky and harbor pixels, not the glyph.
- Adds favicon-16/32/64 PNGs and replaces the old purple-star favicon.svg
  with a lightweight vector split-W on solid navy.
- Wires the new favicons into both the SPA (frontend/index.html) and the
  Symfony Twig base (templates/base.html.twig), replacing the Symfony
  default "sf" emoji data-URL placeholder on the login page.

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

frontend/index.html

@@ -5,7 +5,9 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
     <title>WeVisto</title>
 
-    <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
+    <link rel="icon" type="image/svg+xml" href="/build/favicon.svg" />
+    <link rel="icon" type="image/png" sizes="32x32" href="/build/icons/favicon-32.png" />
+    <link rel="icon" type="image/png" sizes="16x16" href="/build/icons/favicon-16.png" />
     <link rel="manifest" href="/build/manifest.webmanifest" />
     <link rel="apple-touch-icon" href="/build/icons/apple-touch-icon.png" />