New logo + full auth redesign

- Replace purple isometric illustration with a flat brand mark: pink
  gradient tile, ribbed container glyph, ground rail — reads at 32px
- Favicon now uses containr.svg instead of vite.svg
- Auth: drop the split panel for a centered layout — dot-grid, ambient
  pink glow, logo + wordmark above a compact card, real Google mark,
  labeled OAuth divider

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/frontend/index.html

@@ -2,7 +2,7 @@
 <html lang="en" class="dark">
   <head>
     <meta charset="UTF-8" />
-    <link rel="icon" type="image/svg+xml" href="/vite.svg" />
+    <link rel="icon" type="image/svg+xml" href="/containr.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>containr</title>
   </head>