fix(pwa): manifest background_color → navy to kill the white frame

iOS / Chrome use background_color to fill any padding when displaying
the install icon. Cream (#fdf6ee) was leaving a visible 'thick white
border' around the V-viewfinder when the browser auto-padded the icon
on the install-preview screen. Setting background to navy (#0e2740) —
matching the icon's outer fill — makes any auto-padding invisible.

Theme_color also updated for consistency on the install splash.

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

public/build/manifest.webmanifest

@@ -7,8 +7,8 @@
   "scope": "/",
   "display": "standalone",
   "orientation": "any",
-  "background_color": "#fdf6ee",
-  "theme_color": "#fdf6ee",
+  "background_color": "#0e2740",
+  "theme_color": "#0e2740",
   "icons": [
     {
       "src": "/build/icons/icon-192.png?v=20260515-vviewfinder",