assets(flags): replace placeholder flags with realistic designs

Replaces 48 placeholder flag SVGs (simple boxes with country codes) with realistic, detailed flag designs.

assets/images/flags/jpn.svg

@@ -1,5 +1,5 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 60 40" role="img" aria-label="Japan flag">
-  <rect width="60" height="40" rx="4" fill="#f5f5f5"/>
-  <path d="M0 20h60v16a4 4 0 0 1-4 4H4a4 4 0 0 1-4-4z" fill="#bc002d"/>
-  <text x="30" y="25" font-family="Arial, sans-serif" font-size="12" font-weight="700" fill="#ffffff" stroke="#081421" stroke-width="0.8" paint-order="stroke" text-anchor="middle">JPN</text>
-</svg>
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="900" height="600">
+<rect fill="#fff" height="600" width="900"/>
+<circle fill="#bc002d" cx="450" cy="300" r="180"/>
+</svg>