Phase 2: core converter, live conversion, URL state, highlight interaction

src/components/UnitInput.svelte

@@ -0,0 +1,30 @@
+<script lang="ts">
+  /**
+   * UnitInput — controlled numeric input.
+   * Phase 2: number input only. NL field added in Phase 3.
+   */
+
+  let {
+    value,
+    onchange
+  }: {
+    value: number;
+    onchange: (v: number) => void;
+  } = $props();
+
+  function handleInput(e: Event) {
+    const input = e.currentTarget as HTMLInputElement;
+    const parsed = parseFloat(input.value);
+    if (!isNaN(parsed)) {
+      onchange(parsed);
+    }
+  }
+</script>
+
+<input
+  type="number"
+  step="any"
+  value={value}
+  oninput={handleInput}
+  aria-label="Input value"
+/>