docs: redesign README landing page

assets/readme/hero.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1280 560" role="img" aria-labelledby="title desc">
+  <title id="title">U-Claw portable AI workspace</title>
+  <desc id="desc">A USB drive carries a local AI workspace across Windows, macOS, and Linux.</desc>
+  <rect width="1280" height="560" fill="#09121e"/>
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+  <g transform="translate(472 132)">
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+  <g fill="#b8f4e8">
+    <text x="84" y="151" font-family="Arial, sans-serif" font-size="62" font-weight="700">U-CLAW</text>
+    <text x="88" y="202" font-family="Arial, sans-serif" font-size="25" letter-spacing="3">AI THAT TRAVELS WITH YOU</text>
+  </g>
+  <g font-family="Arial, sans-serif" font-size="22" fill="#d8f2e9">
+    <text x="90" y="313">Portable workspace</text>
+    <text x="90" y="350">Local data · Your model · Your USB drive</text>
+  </g>
+  <g transform="translate(923 367)" fill="#d8f2e9" font-family="Arial, sans-serif" font-size="18">
+    <text x="0" y="0">Windows</text><text x="116" y="0">macOS</text><text x="218" y="0">Linux</text>
+  </g>
+</svg>