docs(readme): replace stretched shields.io badge with bordered SVG card

The previous attempt at matching the deer-flow Repository-of-the-Day
design used `style=for-the-badge` shields.io rendered as two wide
black/orange rectangles spanning the README width — nothing like the
small bordered pill deer-flow actually ships (a Trendshift-served badge
with a laurel-wreath "1" emblem and two lines of restrained text).

OpenMontage isn't on Trendshift, so a real Trendshift badge isn't
available. Built a custom inline SVG matching the design intent:

- Small (340x68) bordered card with rounded corners
- Laurel-wreath emblem with serif "1" inside on the left
- Two-line text on the right: "GITHUB TRENDING" (small, dimmed gray)
  and "#1 Repository Of The Day" (bold, accent)
- Light + dark variants served via prefers-color-scheme picture/source
  - Light: white bg, #7C3AED purple stroke/text
  - Dark:  #0D1117 bg, #A78BFA lavender stroke, #C4B5FD headline

Files added:
- .github/assets/repo-of-the-day-light.svg
- .github/assets/repo-of-the-day-dark.svg

README.md

@@ -24,8 +24,8 @@
 <p align="center">
   <a href="https://github.com/trending">
     <picture>
-      <source media="(prefers-color-scheme: dark)" srcset="https://img.shields.io/badge/%F0%9F%8F%86_Repository_of_the_Day-%231_on_GitHub_Trending-F6B543?style=for-the-badge&labelColor=0D1117&color=F6B543">
-      <img alt="🏆 Repository of the Day — #1 on GitHub Trending" src="https://img.shields.io/badge/%F0%9F%8F%86_Repository_of_the_Day-%231_on_GitHub_Trending-D9892A?style=for-the-badge&labelColor=0D1117&color=D9892A" height="48">
+      <source media="(prefers-color-scheme: dark)" srcset=".github/assets/repo-of-the-day-dark.svg">
+      <img alt="🏆 #1 Repository of the Day on GitHub Trending" src=".github/assets/repo-of-the-day-light.svg" height="60">
     </picture>
   </a>
 </p>