feat: avatar component (#82)

src/stories/components/Avatar.mdx

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+import { Meta } from "@storybook/blocks";
+import { UnstyledWithTheme } from "../../components/UnstyledWithTheme";
+
+<Meta title="Components/Avatar" />
+
+# Avatar
+
+<UnstyledWithTheme>
+  <div style={{ display: "flex", flexDirection: 'column', gap: "16px" }}>
+    <div className="avatar">
+      <img src="https://picsum.photos/44" />
+    </div>
+    <div className="avatar">
+      pk
+    </div>
+    <button className="avatar">
+      <img src="https://picsum.photos/44" />
+    </button>
+    <a href="http://google.com" className="avatar">
+      pk
+    </a>
+  </div>
+</UnstyledWithTheme>

src/styles/components/avatar.css

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+:root {
+  --avatar-background: var(--neutral-1);
+  --avatar-foreground: var(--neutral-9);
+  --avatar-border: var(--neutral-2);
+}
+
+.dark-theme {
+  --avatar-background: var(--neutral-9);
+  --avatar-foreground: var(--neutral-1);
+  --avatar-border: var(--neutral-8);
+}
+
+.avatar {
+  --font-scale: 1;
+
+  overflow: hidden;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+
+  width: 44px;
+  height: 44px;
+  padding: 0;
+
+  font-size: calc(var(--font-scale) * var(--font-base-size));
+  font-weight: bold;
+  line-height: var(--line-height-scale);
+  color: var(--avatar-foreground);
+  text-decoration: none;
+  text-transform: uppercase;
+
+  background: var(--avatar-background);
+  border: 1px solid var(--avatar-border);
+  border-radius: var(--corner-5);
+}
+
+.avatar img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+}